Like Haiti, the Belt is dominated by a wealthy class of colonizers (in this case, Earthers who speak English). […] Some characters speak pure Belter, but most speak about half-English, half-Belter, adjusting their vocabulary for each situation. If they’re holding a separatist rally to protest Earther rule, Belter is the order of the day. If they’re trying to talk to a boss, English makes more sense.
Source: wiktionary