When I started twittering I was continously switching between German and English. Then at some point I acknowleged all of my followers were Germans. So I had been twittering in German for months.
Someone sent me a direct message that following me doesn't make sense because the person can't understand my German tweets.
Now twittering in English :-)
P.S.: Some tweets still may be in German if there is local content or translating to English wouldn't make sense.