also, it would be nice if translated posts had a little something to indicate that they are not originally written in this language, but a button to show original might be enough
Feature wise it sounds cool... But if the language tags work fine, it shouldn't be an issue.
Quite often, I see English posts with translate option (I imagine they keep a non English language on but felt comfortable posting in eng? So if I reply in English, you'll end up translating that)