From what I recall from sp characters, they do speak english, but for example, rheinlander soldiers tend to have heavy accent. Like they arent used to speaking english. Also, Tobias speaks english with british accent while Trent who is also bretonian speaks without any accents.
So it might be that nations still use their native languages(look at the dockign bays and bars in kusari, they have kanji all over them, as opposed to english on bretonian and liberty planets) but you either have a translator (that being very wierd because it would only work in space, probably) or english is set as "standard" so everyone speaks it a bit.
