As authors of xili-language trilogy (including xili-tidy-tags and xili-dictionary), we are very proud that Polylang author quotes that he was inspired by xili-language concepts.
Chouby indicates that Polylang has started in September 2011 and writes in FAQ the reasons of his approach.
For WP users, xili-language ( http://wordpress.org/extend/plugins/xili-language/ ) core was public published in feb. 2009 after WP 2.3 appears with his taxonomy library.
Within soon three (3) years, the core of the plugin using taxinomy and .mo switching is based in less then 1000 lines… but for developpers, webmasters, theme's creators and soon newbies, UI and settings lines must be added.
It is obvious that following the releases, lines are now numerous. (and can be, time to time, reduced if WP core provide libraries or functions - especially for admin UI.)
Why three plugins ?
xili-tidy-tags, based on a taxonomy of tags (and not meta of terms using additional DB tables as in PolyLang) is shipped standalone because can be used as semantic tool to group tags (or custom taxonomy) and not only on multilingual website. The XTT data model solves also particular contexts when translation is not on to one or when one term can be used in multiple language. A settings table centralizes the grouping processes.
xili-dictionary can be activated to replace po-edit software to create and translate .po/.mo files. Recently, it uses po mo libraries included in recent WP releases.
If necessary, I will add more infos about the road followed since 3 years with the benefits of the WP evolutions.