Most of the people I follow on Twitter tweet in English, and so do I most of the time.
However, I often retweet stuff written in other languages, and I do also from time to time tweet in Danish and occasionally Spanish myself. This shouldn’t cause any issues for those of my followers who know the same languages as me, but if you only speak English, it must be a tad annoying to see your Twitter stream filling up with gibberish.
In theory I could set up separate Twitter accounts for all the languages I’m likely to tweet in, but that would be a complete mess. Not only would I need to flit back and forwards from one account to another, but it would appear that I had fewer followers than I do, and many people would only follow one of my language personas, even if they would be capable of following more.
I think Twitter should consider adding languages to the user interface, even if it would make it slightly more complex. This would involve adding language capabilities to the user profiles (allowing you to list the languages you can read) and tagging each tweet with a language (presumably everybody would have a default tweeting language). Twitter would then hide tweets written in languages that you cannot read.
I think this would really make life easier for the multilingual twitterers out there.