This isn't really the case, Dregs have plenty of telling signs. They have their slang (which is VERY noticeable in a game that is like 50-99% talking depending on job and roundtype), they have mechanized eyes.
And I feel like this should be the example going forward. Individual factions/groups of people should have some way of knowing who having a deep conversation about their backstory and views. Though linguistic choices could present a bit of a barrier for entry, there are other options - e.g. you can always distinguish a Great House noble by their surname. Or you could have loadout items. Or really all sorts of other readily apparent and visible things.
Very well said. I'm going to make a tattoo of this on my forehead.
The main reason why I'm opposed to this is because of Sol in particular. It's a pretty multilingual place, especially Mars, which has a very heavy connection to Freespeak rather than to Sol Common. I personally have a Martian who only knows Freespeak, but not Common. This connection is also pretty important thematically (showing that the red part of Mars doesn't like Sol all that much, and tries to oppose its culture as well).
Moreover, this kind of buff isn't really significant. We don't have enough languages, or enough uses for languages to justify an additional slot. Skrell, for example, can already pick all languages available. If you give a human Sol Common the additional slot coming with it, they could also select all languages except one (which would be Siik'tau because it's completely irrelevant to anything whatsoever and almost never spoken on station, I await its death eagerly).