Sounds like a nifty Talent, but one that would need to have a high cost -- though this could be mitigated somewhat by limiting it to non-ancient languages (it wouldn't include magical languages anyways), to only current languages. Also, could limit it to only spoken languages and then have a second talent for written languages (i.e. to translate a written language, the character needs to write out the translation, which is done in a trance).
Interesting ideas...