If there’s no one else around speaking English, why cling to the old dialect?
Think about it; if you found yourself in a foreign land at a young age with little possibility of returning home, you would also probably try to assimilate as fast as possible.
Yondu and his band of Ravagers abducted Peter Quill from Earth as a small child, so learning a new alien language probably became a necessity to survive.
Sure there are the alien translators that the Guardians use, but realistically, how many folks are traveling around the galaxy who speak the Earth tongue?