If it's been 400 years, that's not enough time for a new language to develop unless you have actual creolization going on, and in the conditions you're describing, you wouldn't. It seems more likely that you'd wind up with a diglossia situation, with one dominant language used in education and government and spoken to some degree by everyone, and several surviving minority languages whose speakers are almost all bilingual and who use their home languages within the community and the dominant language with outsiders.

The dominant language may have developed into a distinct dialect-- lots of borrowings, quite probably a novel accent, maybe a handful of grammatical quirks-- and of course its divergence from what people are speaking back on Earth will also depend on language change there, since speakers will continue to innovate back at home even after the colonists leave.

Actually, I know you said you didn't just want to have Americans In Space, but the language situation might end up looking an awful lot like the US.