I think in this case, the idea has less to do with irl norms/abilities than it does with abuse/unthemely RP.
Opinion: The two most common languages in the mix should be English and Mash. Topside maybe English and Euro.
A third option that makes thematic sense right in the gate is the character's native language.
So depending where you spend the most time, that's three thematically acceptable languages as a standard.
An stat gate would prevent players from pursuing every language in order to "know what everyone is saying all the time". In terms of RP, I think this kind of mentality should mostly be reserved primarily to archetypes that require it, and for characters that specifically commit to x stats for this reason.
Fixers, scientists, public personalities, syndicate employees. Things like that.
In this regard, that opens up the option for a variety of stats to pursue for this goal. Intelligence, charisma, artistry, trading, to name a few.
A few other thoughts on this to make the language system more interesting/relevant/robust:
1. Mash is a pidgin language. I.E. a conglomerate developed between groups without a common language. Mixers therefore, should get a bonus to learning multiple languages beyond the first three (English, Mash, Native). This reflects the immersive nature of the melting pot that is the mix.
2. Topside dwellers should get negatives to languages beyond their first three (English, Euro, Native). This reflects the elitist community topside in which the lingua franca is English, and you might even be looked down on for speaking something else. Maybe it's rude. Maybe it comes off as secretive and sleazy in public. Maybe it's lowbrow. Maybe it's overtly shady to hide your conversations in public. That's mixer shit, you degenerate scumbag. Scheme behind closed doors and in sic like a real corpo, baka. Immersion is much lessened as a result, and people don't learn other languages as easily.
3. Syndicate members receive a bonus to the primary language of that faction. Cumulative with mix immersion bonus.