how should I prepare a link like  [[:en:UTF-8|UTF-8]] with tvar ?

The easy way is [[<tvar|link>:en:UTF-8</>|UTF-8]], I guess.

But it would be better if the link would lead to the localized wikipedia article if it does exist, but fallback on the original otherwise. Such a behaviour is in fact rather tricky to perform, and I'm little stuck there, trying to misc. combination of "{{#translation:}}" and "Special:MyLanguage/". Maybe some magic keyword jedi master might help here? :)