Federation standard is not the language that the average Federation member uses at home, unless that home is a Human or Tellerite world, where it is the preferred language in schools. Worlds like Vulcan and Andor, among others, teach it as a second language preserving local tongues.
Federation Standard sounds remarkably like 20th Century American English, at least on the Tee Vee and in Movies. That might have something to do with the intended audience. It is also noted that all just contacted aliens speak it flawlessly, unless that is a McGuffin in the episode.