In Drupal 6 you had 2 way to support multilingual taxonomy :
1) a master monolingual taxonomy with internationalization done by i18n module :
example of use : $termslg=tt('taxonomy:term:'.$term->tid.':name',$term->name);
Problem : only one master monolingual repository use in input form and no good environment to check and maintain translation of term for large list
2) a full parallel multilingual taxonomy system with term existing for each language
example of use : $termslg=i18ntaxonomy_translate_terms($terms, $lang);
Problem : each document are related to term in its own language and no standard support exists to support multilingual search based on the language synonym relation. That is e.g. a problem for "active translation" environment when you show to user all documents even if it is not translate
Proposal :
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Implement model 2) to support in standard definition of taxonomy search rules take into account extraction of all document whatever the language is (use of language synonym relation)
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If model 1) is kept and not replace by upgrated model 2), develop an editorial interface allowing maintenance and control of translation for large list of terms

Comments
Hi orfils, good summary. The
Hi orfils, good summary.
The problem with all this is that these are wrappers on top of Drupal taxonomy system which has no concept of language at all. So it's already complex enough.
But anyway, whatever implementation of what you suggest for search would be great, not that I have the time to do it. However keep in mind that options are per-vocabulary so both models can coexist in a site, such search will be complex to do.
FYI we are having a BoF session on multilingual search in Paris in case you are around, http://paris2009.drupalcon.org/session/multi-language-search