I talked about language detection (language identification) for twitter at NAIST(NARA Institute of Science and Technology).
This is its slide.
Tweets are too short to detect their languages precisely. I guess that one reason is because features extracted from a short text are not enough to detect.
Another reason is because tweets have some unique representations, for example, u as you, 4 as for, LOL, F4F, various face marks and so on.
I developed ldig, a prototype of short text language detection, that solved those problems.
ldig can detect langages of tweets with over 99% accuracy for 19 languages.
The above slide explains how ldig solves those problems.