The metadata, that sits in every tweet contains no locale definition. If it did, Twitter clients (including Twitters website) could filter tweets by language, and hide me from Chinese, Spanish and other very nice languages … that I nevertheless don’t understand.
Update: As George Hahn points out in the comments, Twitter basically supports tweet languages. This means apps like Tweetbot or Osfoora could give options like “don’t show Tweets that are tagged as Chinese or Japanese”. /Update
Putting in the language would then of course be an additional hassle for multiple language tweeters, but effectively it would do no more harm that what is done right now to the unprotected eyes of usual people who want to try twitter but might be scared away if they get all the english retweets – while they don’t speak english. Then also applications can try to auto recognize if I tweet in german or english (which might not even be that hard) and correct me if I didn’t put in the language manually.