Thread:Talk:Universal Language Selector/Is there some expression that evaluates to the current language?/reply

No, there isn't. The request to add one has been rejected/delayed many times because it would allegedly break Wikimedia projects' caching system, see 2085. Wikis are using a hack which achieves the same effect anyway (so allegedly breaking thier cache, but nobody cares), that is a custom message Lang called with int magic word, which will return the language code of the user's interface. I can't tell if it's the appropriate solution for you, but you can do the same or locally hack your MediaWiki installation as e.g. translatewiki.net does (I think some patches were published on the bug or elsewhere).