Istituto di Linguistica Computazionale
Consiglio Nazionale delle Ricerche, Pisa, Italy
With the recent rapid diffusion over the international computer networks of world-wide distributed document bases, the question of multilingual access and multilingual information retrieval is becoming increasingly relevant. We briefly discuss just some of the issues that must be addressed in order to implement a multilingual interface for a Digital Library system and describe our own approach to this problem.
So far, in the Digital Library (DL) sector, most research and development activities have concentrated on monolingual environments and, in the large majority of cases, the language employed has been English.
This is understandable for two reasons: The first DL efforts were concentrated in the United States where English is generally accepted as the default language, and development thus focused on other strategic areas. Second, until very recently the international information infrastructures and advanced communication and information access services had not attempted to address the technical difficulties of creating Internet or Web applications that successfully span multiple languages. However, the scene is rapidly changing.
Over the last few years, we have seen an enormous growth of interest in the construction of digital library systems throughout the world, and not just in mainly English speaking areas. Both Asia and Europe are now actively involved in building up their own large distributed organised repositories of knowledge. This was witnessed by a recent number of Ercim News, the newsletter of the European Research Consortium for Informatics and Mathematics, which was dedicated to Digital Libraries and described on-going initiatives throughout Europe and also in China and Japan. It is also shown by the number of international conferences now being organized outside of the United States. Important DL Conferences have already been held in Japan; this autumn, the very first European Conference on Research and Advanced Technology for Digital Libraries, sponsored by the European Union, will be held in Pisa, Italy, 1-3 September.
Thus an increasing amount of the world's knowledge is being organized in domain-specific compartments and stored in digital form, accessible over the Internet. Not only are monolingual digital libraries being created in many different languages, but multilingual digital libraries are becoming more common, for example, in countries with more than one national language, in countries where both the national language and English are commonly used for scientific and technical documentation, in pan-European institutions such as research consortia, in multinational companies, and so on. We must now begin to take measures to enable global access to all kinds of digital libraries, whether mono- or multilingual, and whatever the language of their content.
This is not a trivial question. We are talking about increasing the world-wide potential for access to knowledge and implicitly to progress and development. Not only should it be possible for users throughout the world to have access to the massive amounts of information of all types -- scientific, economic, literary, news, etc. -- now available over the networks, but also for information providers to make their work and ideas available in their preferred language, confident that this does not in itself preclude or limit access. This is particularly relevant for the "non-dominant" languages of the world, i.e. not English, Japanese and a few of the major European languages, but most of the other languages. The diversity of the world's languages and cultures gives rise to an enormous wealth of knowledge and ideas. It is thus essential that we study and develop computational methodologies and tools that help us to preserve and exploit this heritage. The survival of languages which are not available for electronic communication will become increasingly problematic in the future.
That this is a strategically important issue has recently been recognised by a programme for European and US cooperation on digital library research and development sponsored by the European Union and the National Science Foundation. The original programme defined the setting up of just four working groups to discuss and explore jointly technical, social and economic issues, to co-ordinate research (where it makes sense), and to share research results in the areas of: interoperability; metadata; search and retrieval; intellectual property rights and economic charging mechanisms. However, it has now been decided to add a fifth working group to this list in order to investigate issues regarding multilinguality. For more information on this programme, see the Web site of the ERCIM Digital Library Initiative.
Unfortunately, the question of multilingual access is an extremely complex one. Two basic issues are involved:
The first point addresses the problem of allowing DL users to access the system, no matter where they are located, and no matter in what language the information is stored; it is a question of providing the enabling technology.
The second point implies permitting the users of a Digital Library containing documents in different languages to specify their information needs in their preferred language while retrieving documents matching their query in whatever language the document is stored; this is an area in which much research is now under way.
Before we go on to discuss these topics in more detail in the next two sections, let us define some relevant terms:
2. Multilingual Recognition and Representation
Despite its name, until recently the World Wide Web had not addressed one of the basic challenges to global communication: the multiplicity of languages. Standards for protocols and document formats originally paid little attention to issues such as character encoding, multilingual documents, or the specific requirements of particular languages and scripts. Consequently, the vast majority of WWW browsers still do not support multilingual data representation and recognition. Ad-hoc local solutions currently abound which, if left unchecked, could lead to groups of users working in incompatible isolation.
The main requirements of a multilingual application are to:
In the following we briefly mention some of the measures now being taken to provide features for internationalization, i.e. multilingual support, on the Web in the core standards: HTTP (HyperText Transfer Protocol) and HTML (HyperText Markup Language). Support of this type is essential to enable world-wide access both to digital libraries containing documents in languages other than English, and to multilingual digital libraries.
For fuller information, the reader is encouraged to refer to the list of useful URLs at the end of this section, and to , .
HTTP is the main protocol for the transfer of Web documents and resources. It thus provides meta-information about resources and content-negotiation. Features for tagging and client-server negotiation of character encoding and language were first included in HTTP 1.1 (RFC 2068).
The character encoding of the document is indicated with a parameter in the Content-Type header field.
For example, to indicate that the transmitted document is encoded in the "JUNET" encoding of Japanese,
the header will contain the following line:
Content-type: text/html; charset=iso-2022-JP.
Content-Language is used to indicate the language of the document. The client can indicate both preferred character encodings (Accept-Charset) and preferred language (Accept-Language). Additional parameters can be used to indicate relative preference for different character encodings and, in particular, for different languages, in a content negotiation. So, for example, a user can specify a preference for documents in English but indicate that French, Spanish and German are also acceptable.
RFC 2070 adds the necessary features to HTML for describing multilingual documents and for handling some script or language specific features that require additional structure. These additions are designed so that they extend to new versions of HTML easily. RFC 2070 is now the proposed standard to extend HTML 2.0 (RFC 1886), primarily by removing the restriction to the ISO-8859-1 coded character set.
Initially the application of HTML was seriously restricted by its reliance on the ISO-8859-1 coded character set (known as Latin-1), which is appropriate only for Western European languages. Latin-1 has 8-bit encoding which permits a maximum of just 256 characters. Despite this restriction, HTML has been widely used with other languages, using other character sets or character encodings, at the expense of interoperability. For example, several 8-bit ISO standard character sets can be adopted to cover the set of languages being treated; the document metadata will include information on the character code used in that document. This is perhaps feasible as long as coverage is limited to the most common European languages. The problem becomes much more complex, however, if we want to start moving between, for example, French and Arabic, English and Japanese. If we start to use a large number of character sets and encodings, and if the browser is to handle translation from one set to another, the system response times will be heavily affected.
For this reason, the internationalization of HTML by extending its specification is essential. It is important that HTML remains a valid application of SGML, while enabling its use with all the languages of the world. The document character set in the SGML sense is the Universal Character Set (UCS) of ISO 10646:1993. Currently, this is code-by-code identical with the Unicode standard version 1.1. ISO 10646/Unicode has thus been chosen as the document character set with the main consequence that numeric character references are interpreted in ISO 10646 irrespective of the character encoding of the document, and the transcoding does not have to know anything about SGML syntax.
The Unicode Character Standard is a single 16-bit character encoding designed to be able to represent all languages. Unicode encodes scripts (collections of symbols) rather than languages. 16-bits permit over 65,000 characters. It currently contains coded characters covering the principal written languages of the Americas, Europe, Middle East, Africa, India, Asia. Unicode characters are language neutral. A higher level protocol must be used to specify the language. Although a 16-bit code ensures that a document can be displayed without relying on its metadata, it places higher demands on storage, and could significantly affect long distance transmittal times. This is the reason why there has been considerable resistance to the idea of the universal adoption of Unicode.
However, this reluctance to employ Unicode is destined to gradually fall away as the advantages of global language interoperability are found to far outweigh the trade-off in heavier storage requirements and the potential effect on response times. The fact that Netscape has decided to design future products to support the Unicode standard and also to add functionalities to assist users in multilingual applications will certainly play a considerable role. For example, content creators will be able to store multiple versions of a document in different languages behind one URL; documents will be allowed to contain text in multiple languages on the same page; the development tools will be made language independent.
Another important feature for multilinguality introduced by RFC 2070 is the language attribute (LANG) which can be included in most HTML elements. It takes as its value a language tag that identifies a written or spoken natural language and serves to indicate the language of all or of a certain part of a document. The values for the language tags are specified in RFC 1766. They are composed of a primary tag and one or more optional subtags, e.g. en, en-US, en-cockney, and so on.
The rendering of elements may be affected by the LANG attribute. For any element, the value of LANG overrides the value specified by the LANG attribute of any enclosing element and the value (if any) of the HTTP Content-Language header. This information can be used for classification, searching and sorting, and to control language dependent features such as hyphenation, quotation marks, spacing, ligatures. It is highly recommended that document providers should include this attribute in the header information, otherwise some automatic language control may be needed for a digital library cross-language query system.
With the document character set being the full ISO 10646, the possibility that a character cannot be displayed locally due to lack of appropriate fonts cannot be avoided. Provisions to handle this situation must be supplied by the local application.
There are many factors that affect language-dependent presentation. For example, there is a wide variation in the format and units used for the display of things like dates, times, weights, etc. This problem will have to be addressed eventually rather than leaving it for local solutions. A proposal is made in .
RFC 2070 introduces HTML elements to support mark-up for the following features:
Other protocols and resources, such as FTP, URLs, and domain names are being worked on with respect to multiscript support. The chosen solution is UTF-8, a fully ASCII-compatible variable-length encoding of ISO 10646/Unicode.
To sum up, it is probably true to say that the base facilities for multilingual applications running on the WWW are now in place. Such applications should take advantages of these facilities and contribute to their spread and better use. Even monolingual Digital Libraries should include the relevant features if they want to guarantee their global accessibility.
Acknowledgement: This section owes much to a presentation by Martin Duerst at a recent workshop held in Zurich, Switzerland: Third DELOS Workshop on Multilingual Information Retrieval.
For information on the activities of WInter: Web Internationalization & Multilinguism -- http://www.w3.org/pub/WWW/International/
For references and direct links to the protocols, standards and Internet drafts mentioned in this section and many others -- http://home.netscape.com/people/erik/internet- intl.html
For further information on Unicode -- http://www.unicode.org/
To keep in touch with developments in this area, the reader is advised to subscribe to the mailing list (www-
email@example.com) created for discussing internationalization on the Web.
3. Cross-Language Retrieval
As explained in the previous section, the interface of a multilingual Digital Library must include features to support all the languages that will be maintained by the system and to permit easy access to all the documents contained. However, it must also include functionalities for multilingual or cross-language search and retrieval. This implies the development of tools that allow users to interact with the system, formulating the queries in one language and retrieving documents in others. The problem is to find methods which successfully match queries against documents over languages. This involves a relatively new discipline, generally known as Cross-Language Information Retrieval (CLIR), in which methodologies and tools developed for Natural Language Processing (NLP) are being integrated with techniques and results coming from the Information Retrieval (IR) field.
There has been much interest in this emerging area over the last year and a number of important international workshops have been held. We will cite just two of them: the Workshop on Cross-Linguistic Information Retrieval held at SIGIR '96 in Zurich, and the Workshop on Cross-Language Text and Speech Retrieval held at the AAAI-97 Spring Symposium Series, in Stanford, this spring. An important overview of recent work is given in 
Three main approaches to CLIR have been identified:
Each of these methods has shown promise but also has disadvantages associated with it. We will briefly outline here below some of the main approaches that have been or are now being tried. Unfortunately, lack of space means that it is impossible to go into much detail or attempt to give an exhaustive list of the current activities in this area.
Research has thus concentrated on finding ways to translate the query into the language(s) of the documents. Performing retrieval before translation is far more economic than vice versa: generally only a small percentage of documents in a collection are of wide interest; it is only necessary to translate those documents retrieved that are actually found to be of interest; users frequently have sufficient reading ability in a language for adequate comprehension although they would not have been capable of formulating a correct query.
An exception to this rule is the TwentyOne project which combines a (partial) document translation (DT) with query translation. The main approach is the document translation -- using both full MT translations and term-translation as a fall-back option -- as DT can fully exploit context for disambiguation whereas, it is well-known that the average query is too short to permit resolution of ambiguous terms. The database consists of documents in a number of languages, initially Dutch, French and German but extensions to other European languages are envisaged. Presumably it is the fact that the project covers a fairly restricted area that makes the idea of document translation feasible. This approach would appear to have severe scalability problems as more languages are included.
Using Dictionaries: Some of the first methods attempting to match the query to the document have used dictionaries. It has been shown that dictionary-based query translation, where each term or phrase in the query is replaced by a list of all its possible translations, represents an acceptable first pass at cross- language information retrieval although such -- relatively simple -- methods clearly show performance below that of monolingual retrieval. Automatic machine readable dictionary (MRD) query translation has been found to lead to a drop in effectiveness of 40-60% of monolingual retrieval , . There are three main reasons for this: general purpose dictionaries do not normally contain specialised vocabulary; the presence of spurious translations; failure to translate multiword terms.
Fluhr et al  have reported considerably better results with EMIR (European Multilingual Information retrieval). EMIR has demonstrated the feasibility of a cross-language querying of full-text multilingual databases, including interrogation of multilingual documents. It uses a ranked boolean retrieval system in conjunction with bilingual term, compound, and idiom dictionaries for query translation and document retrieval. It should be noted that a domain dependent terminology dictionary and extensive manual editing are needed to achieve this performance. However, it is claimed that little work is needed to adapt the dictionaries when processing a new domain, and tools have been developed to assist this process. The technology was tested on three languages: English, French and German. Part of the EMIR results have already been introduced into the commercial cross-language text retrieval system known as SPIRIT. Another working system -- if still primitive according to the developers -- that uses a bilingual dictionary to translate queries from Japanese to English and English to Japanese is TITAN . TITAN has been developed to assist Japanese users to explore the WWW in their own language. The main problems found are those common to many other systems: the shortness of the average query and thus the lack of contextual information for disambiguation, the difficulty of recognizing and translating compound nouns.
Recent work is attempting to improve this basic performance. David Hull , for example, describes a weighted boolean model based on a probabilistic formulation in order to help to solve the problem of target language ambiguity. However, this model relies on relatively long queries and considerable user interaction, while real world tests show that users tend to use over short queries and shy away from any form of user-system dialogue.
Ballesteros and Croft  show how query expansion techniques using pre- and post-translation local context analysis can significantly reduce the error associated with dictionary translation and help to translate multi-word terms accurately. However, as dictionaries do not provide enough context for accurate translations of most types of phrases, they are now investigating whether the generation of a corpus-based cross-language association thesaurus would provide enough context to resolve this problem.
Using Thesauri: The best known and tested approaches to CLIR are thesaurus-based. A thesaurus is an ontology specialised in organising terminology; a multilingual thesaurus organizes terminology for more than one language. ISO 5964 gives specifications for the incorporation of domain knowledge in multilingual thesauri and identifies alternative techniques. There are now a number of thesaurus-based systems available commercially. However, although the use of multilingual thesauri has been shown to give good results for CLIR -- early work by Salton  demonstrated that cross-language systems can perform as well as monolingual systems given a carefully constructed bilingual thesaurus -- thesaurus construction and maintenance is expensive, and training is required for optimum usage.
Dagobert Soergel  discusses how in information retrieval a thesaurus can be used in two ways:
A multilingual thesaurus for indexing and searching with a controlled vocabulary can be seen as a set of monolingual thesauri that all map to a common system of concepts. With a controlled vocabulary, there is a defined set of concepts used in indexing and searching. Cross-language retrieval means that the user should be able to use a term in his/her language to find the corresponding concept identifier in order to retrieve documents. In the simplest system, this can be achieved through manual look-up in a thesaurus that includes for each concept corresponding terms from several languages and has an index for each language. In more sophisticated systems, the mapping from term to descriptor would be done internally.
The problem with the controlled vocabulary approach is that terms from the vocabulary must be assigned to each document in the collection. Traditionally this was done manually. Methods are now being developed for the (semi-)automatic assignation of these indicators. Another problem with this method is that it has been found to be quite difficult to train users to effectively exploit the thesaurus relationships.
Cross-language free-text searching is a more complex task. It requires that each term in the query be mapped to a set of search terms in the language of the texts, possibly attaching weights to each search term expressing the degree to which occurrence of a search term in a text would contribute to the relevance of the text to the query term. Soergel explains that the greater difficulty of free-text cross-language retrieval stems from the fact that one is working with actual usage while in controlled-vocabulary retrieval one can, to some extent, dictate usage. However, the query potential should be greater than with a controlled vocabulary.
Using Ontologies: The only general-purpose multilingual ontology that we know anything about is that being developed in the EuroWordNet project. EuroWordNet is a multilingual database which represents basic semantic relations between words for several European languages (Dutch, Italian, Spanish and English) taking as its starting point Princeton WordNet 1.5. For each of the languages involved, monolingual wordnets are being constructed maintaining language-specific cultural and linguistic differences. All the word-nets will share a common top-ontology and multilingual relations will be mapped from each individual wordnet to a structure based on Wordnet 1.5 meanings. Such relations will form an Interlingual Index. The EWN database is now being tested as a resource to perform cross-language conceptual text retrieval. Unfortunately, no results are available yet. .
The main problems with thesauri and ontologies are that they are expensive to build, costly to maintain and difficult to update. Languages differences and cultural factors mean that it is difficult to achieve an effective mapping between lexical or conceptual equivalences in two languages; this problem is greatly exacerbated when several languages are involved. It is necessary to build some kind of interlingua to permit transfer over all languages; it is to be expected that the trade-off for multilinguality will be the loss of some monolingual specificity.
These considerations have encouraged an interest in corpus-based techniques in which information about the relationship between terms is obtained from observed statistics of term usage. Corpus-based approaches analyse large collections of texts and automatically extract the information needed to construct application- specific translation techniques. The collections analysed may consist of parallel (translation equivalent) or comparable (domain-specific) sets of documents. The main approaches that have been experimented using corpora are vector space and probabilistic techniques.
The first tests with parallel corpora were on statistical methods for the extraction of multilingual term equivalence data which could be used as input for the lexical component of MT systems. Some of the most interesting recent experiments, however, are those using a matrix reduction technique known as Latent Semantic Indexing (LSI) to extract language independent terms and document representations from parallel corpora , . LSI applies a singular value decomposition to a large, sparse term document co-occurrence matrix (including terms from all parallel versions of the documents) and extracts a subset of the singular vectors to form a new vector space. Thus queries in one language can retrieve documents in the other (as well as in the original language). This method has been tested with positive results on parallel text collections in English with French, Spanish, Greek and Japanese.
The problem with using parallel texts as training corpora is that test corpora are very much domain specific and costly to acquire -- it is difficult to find already existing translations of the right kind of documents and translated versions are expensive to create. For this reason, there has been a lot of interest recently in the potential of comparable corpora. A comparable document collection is one in which documents are aligned on the basis of the similarity between the topics they address rather than because they are translation equivalent. Sheridan and Ballerini  report results using a reference corpus created by aligning news stories from the Swiss news agency (SDA) in German and Italian by topic label and date and then merging them to build a "similarity thesaurus". German queries were then tested over a large collection of Italian documents. They found that the Italian documents were retrieved with a better effectiveness than with a baseline system evaluating Italian queries against Italian documents. They claim that this is a result of the query expansion method used as the query is padded with related terms from the document collection. However, although this means that their recall performance is high, their precision level is not so good. Although this method is interesting and the results reported positive, its general applicability remains to be demonstrated. The collection used to build the multilingual similarity thesaurus was the same as that on which the system was tested.
Again, as with the parallel corpus method reported above, it appears that this method is very application dependent. A new reference corpus and similarity thesaurus would have to be built to perform retrieval on a new topic; it is also unclear how well this method can adapt to searching a large heterogeneous collection.
The current trend seems to be to experiment with a combination of more than one method, i.e. to use a combination of dictionaries or thesauri, corpora and/or user interaction. A very good example of this is the work at NEC where Yamabana et al  have built an English/Japanese retrieval system which uses bilingual dictionary, comparable corpora, and then user interaction in order to enhance performance. The retrieved documents are passed through a machine translation system before being sent to the user.
At the present moment, we feel that the most promising and cost-effective solution for CLIR within the DL paradigm will probably be an integration of a multilingual thesaurus with corpus-based techniques. In the next section, we will discuss the strategy we are now studying. We believe that it should be possible to overcome the problem of the ad hoc construction of a suitable training corpus in a multilingual digital library by using the digital library itself as the source of training data.
Useful URLs: For details of current work in Multilingual Text Retrieval, an excellent Web site is: http://www.ee.umd.edu/medlab/mlir/mlir.html, maintained by Doug Oard, University of Maryland.
Go to Section 4. Our Approach
Go to Section 5. References