I'm pretty sure Twitter's langid uses character n-grams. You'll see a tweet that's plain English that happens to match n-gram statistics unusually well with som

I'm pretty sure Twitter's langid uses character n-grams. You'll see a tweet that's plain English that happens to match n-gram statistics unusually well with some other language, which pushes the likelihood score to just above English. (I checked this by running an example or two through my own langid code.) It shouldn't be hard to improve on by treating the score on a tweet as bayesian evidence to combine with a prior from preceding tweets.