I've always wanted to know if we are "comfortable in English" or "comfortable with English". I have searched quite a few websites but none are helpful when it comes to suggesting a preposition for speaking in a language.
Lingohelp gives a corpus-based overview of the idiomaticity of 'comfortable with' and 'comfortable in'. One of the examples is spot-on:
So, I am more comfortable with Hindi than I am with Tamil.
However, a lot of the 'comfortable in' examples seem to involve locative prepositional phrases such as 'I felt comfortable in their house', which skew the overall picture. Raw data doesn't really reflect the actual frequency of occurrence of the colligation 'comfortable in'. That said, the popular choice is 'comfortable with', and it would be my choice here (for the bald statement, with no context).
I'd possibly choose 'I'm comfortable in English ...' if someone asked 'Would you prefer to continue this conversation in German?', so I don't think either choice is unacceptable per se.