I think that they will speak true Terran Standard.
It would be similar to English, but not the one we speak now, just look at modern slang, now imagine it became standard English, then there was slang diverted from it, then repeat it a few times. Or imagine that for some reason there was a large-scale emigration from, for example, Italy to England at some point in history, it would certainly have influence on language, as English will be modified by some Italian divertives. Also, world is getting smaller as globalization is going on, so languages have a lot of opportunities to mix and affect each other.
Now imagine it going for 300 years, the English we use now would be rendered archaic and used only in some books happening in our century.
Just check language which was in use 300 years ago.