I'd try System Restore to yesterday.
Having a recent image on another partition or disk comes in very handy also. It's more reliable than system restore. But not everybody uses such precautions.
I've been using this computer for almost 4 years, switching between Thai and English using the language bar.
As of this morning, when it says 'TH' in the language bar, it types in English rather than Thai.
The same thing happens when it's set to Korean (I don't speak Korean, but bought the laptop over there, so it's installed, and I tried it to check).
I can change between the languages displayed using Alt + Shift or by clicking on the menu, but the output when I type is always in the Roman alphabet.
Any ideas what's happening and how I can fix it?