I'm looking for research on how Twitter users negotiate which language to use when tweeting. Does their choice of language depend on the topic of the tweet? Does it depend on their audience?
No, I don't. Maybe they use a language like english to improve their followers. Sometime they use one or other language depending on what they want to tell. I thibk this two options are correlated.
Me, I use spanish to talk to people I know more; and other languages depending on the public (if they talk to me in their languages I answer in it) and on the topic. This last point, it depends if it is an specific topic that has something related to a country, like its history or culture; if I want to get more other audience, I write in two or more languages at the same time.
As to what language people use on twitter seems to be partly dependent on a) what they normally use, b) what they are proficient at and c) which language is spoken by the majority of their intended audience
There is some ongoing research about codeswtiching and bilingualism on Twitter in the Phillipines - http://www.glennabastillas.com/
And I have done research on language use on twitter in Berlin, Germany. However, without a qualitative study into the factors behind people's language choices, I can only make educated guesses into this choice. I would agree that it is audience dependent. As all Tweets are public, it is difficult to determine who one's intended audience might be.