Apertium

Apertium is a free/open-source rule-based machine translation platform. It is free software and released under the terms of the GNU General Public License.

Overview 
Apertium is a transfer-based machine translation system, which uses finite state transducers for all of its lexical transformations, and Constraint Grammar taggers as well as hidden Markov models or Perceptrons for part-of-speech tagging / word category disambiguation. A structural transfer component is responsible for word movement and agreement; most Apertium language pairs up until now have used "chunking" or shallow transfer rules, though newer pairs use (possibly recursive) rules defined in a Context-free grammar.

Many existing machine translation systems available at present are commercial or use proprietary technologies, which makes them very hard to adapt to new usages. Apertium code and data is free software and uses a language-independent specification, to allow for the ease of contributing to Apertium, more efficient development, and enhancing the project's overall growth.

At present (December 2020), Apertium has released 51 stable language pairs, delivering fast translation with reasonably intelligible results (errors are easily corrected). Being an open-source project, Apertium provides tools for potential developers to build their own language pair and contribute to the project.

History 
Apertium originated as one of the machine translation engines in the project OpenTrad, which was funded by the Spanish government, and developed by the Transducens research group at the Universitat d'Alacant. It was originally designed to translate between closely related languages, although it has recently been expanded to treat more divergent language pairs. To create a new machine translation system, one just has to develop linguistic data (dictionaries, rules) in well-specified XML formats.

Language data developed for it (in collaboration with the Universidade de Vigo, the Universitat Politècnica de Catalunya and the Universitat Pompeu Fabra) currently support (in stable version) the Arabic, Aragonese, Asturian, Basque, Belarusian, Breton, Bulgarian, Catalan, Crimean Tatar, Danish, English, Esperanto, French, Galician, Hindi, Icelandic, Indonesian, Italian, Kazakh, Macedonian, Malaysian, Maltese, Northern Sami, Norwegian (Bokmål and Nynorsk), Occitan, Polish, Portuguese, Romanian, Russian, Sardinian, Serbo-Croatian, Silesian, Slovene, Spanish, Swedish, Tatar, Ukrainian, Urdu, and Welsh languages. A full list is available below. Several companies are also involved in the development of Apertium, including Prompsit Language Engineering, Imaxin Software and Eleka Ingeniaritza Linguistikoa.

The project has taken part in the 2009, 2010, 2011, 2012, 2013 and 2014 editions of Google Summer of Code and the 2010, 2011, 2012, 2013, 2014, 2015, 2016 and 2017 editions of Google Code-In.

Translation methodology 

This is an overall, step-by-step view how Apertium works.

The diagram displays the steps that Apertium takes to translate a source-language text (the text we want to translate) into a target-language text (the translated text).
Source language text is passed into Apertium for translation.
The deformatter removes formatting markup (HTML, RTF, etc.) that should be kept in place but not translated.
The morphological analyser segments the text (expanding elisions, marking set phrases, etc.), and looks up segments in the language dictionaries, returning dictionary forms and tags for all matches. In pairs that involve agglutinative morphology, including a number of Turkic languages, a Helsinki Finite State Transducer (HFST) is used. Otherwise, an Apertium-specific finite state transducer system called lttoolbox, is used. 
The morphological disambiguator (the morphological analyser and the morphological disambiguator together form the part of speech tagger) resolves ambiguous segments (i.e., when there is more than one match) by choosing one match. Apertium uses Constraint Grammar rules (with the vislcg3 parser) for most of its language pairs.
 Retokenisation uses a finite state transducer to match  sequences of lexical units and may reorder or translate tags (often used for translating idiomatic expressions into something that more approaches the target language grammar)
Lexical transfer looks up disambiguated source-language basewords to find their target-language equivalents (i.e., mapping source language to target language). For lexical transfer, Apertium uses an XML-based dictionary format called bidix.
Lexical selection chooses between alternative translations when the source text word has alternative meanings. Apertium uses a specific XML-based technology, apertium-lex-tools, to perform lexical selection.
Structural transfer (i.e., it is an XML format that allows writing complex structural transfer rules) can consist of one-step chunking transfer, three-step chunking transfer or a CFG-based transfer module. The chunking modules flag grammatical differences between the source language and target language (e.g. gender or number agreement) by creating a sequence of chunks containing markers for this. They then reorder or modify chunks in order to produce a grammatical translation in the target-language. The newer CFG-based module matches input sequences into possible parse trees, selecting the best-ranking one and applying transformation rules on the tree.
The morphological generator uses the tags to deliver the correct target language surface form. The morphological generator is a morphological transducer, just like the morphological analyser. A morphological transducer both analyses and generates forms.
The post-generator makes any necessary orthographic changes due to the contact of words (e.g. elisions).
The reformatter replaces formatting markup (HTML, RTF, etc.) that was removed by the deformatter in the first step.
Apertium delivers the target-language translation.

Language pairs 
List of currently stable language pairs, hover over the language codes to see the languages that they represent.

See also 

Babel Fish (discontinued; redirects to main Yahoo! site)
Comparison of machine translation applications
Jollo (discontinued)
Microsoft Translator
Moses
OpenLogos
SYSTRAN
Yandex.Translate

Notes

References 
Corbí-Bellot, M. et al. (2005) "An open-source shallow-transfer machine translation engine for the romance languages of Spain" in Proceedings of the European Association for Machine Translation, 10th Annual Conference, Budapest 2005, pp. 79–86
Armentano-Oller, C. et al. (2006) "Open-source Portuguese-Spanish machine translation" in Lecture Notes in Computer Science 3960 [Computational Processing of the Portuguese Language, Proceedings of the 7th International Workshop on Computational Processing of Written and Spoken Portuguese, PROPOR 2006], p 50–59.
Forcada, M. L. et al. (2010) "Documentation of the Open-Source Shallow-Transfer Machine Translation Platform Apertium" in Departament de Llenguatges i Sistemes Informatics, University of Alacant.
Forcada, M. L. et al. (2011) "Apertium: a free/open-source platform for rule-based machine translation". in "

External links 
Apertium home
Apertium Wiki
OpenTrad

End-user services and software 
(All services are based on the Apertium engine)

Online translation websites 
Apertium Translation home
Prompsit Translator 
PoliTraductor Translator
University d' Alacant Translator
Universitat Oberta de Catalunya Translator

Offline applications 
Apertium Caffeine
Apertium Android
Apertium OmegaT

Free software programmed in C++
Machine translation software
Natural language processing software
Natural language processing toolkits
Products introduced in 2009
Translation websites