Realistically, Common would probably be, as stated by some people previously, closer to American English most of all (on account of WotC being based in US), so any localization attempts will be secondary in their interpretations based on that very aspect. Therefore, what it lingustically supports is implied to be, for the most part, whatever English allows. Meanwhile something like Das Schwarze Auge is German by default, with most explored/"default" cultures rooted in the medieval Central/Western Europe and the use of the language is meant to reflect that.

Whereas with the state of the FR I agree with Sozz in that it is meant to be marketable and "modern", what with WotC being a big name company and all, so alterations are made to "keep up with the times", as it were. The current idea of how the setting works and what it supports, despite the fact that culturally it essentially got a rewind after the Second Sundering (5e was specifically meant as practically a "never mind, let's go back to how things were" after 4e's fiasco). I touched on those (the cultural aspects) in the opening post, and yes - despite it being almost a good century and a half between the pre-Spellplague and the modern days, most communities still function and think with the same mindsets as back then, which could be seen as a way of coping with the world finally returning to its pre-cataclysmic state. If anything, there are even cases of reduced acceptance and tolerance, reflected in the game (the Elturel tieflings post-Descent). If anything, Larian are pushing the envelope as to what WotC would probably consider "safe" with their writing and content.

Basically, the modern presentation of the FR is as much a product of its time as the attempts to market it to teens and young adults operating upon the trends of the early 2000's and the turn of the decade - take the M:tG artwork from those eras to see what they were considering marketable (cleavages, anime-ish artstyles, attempts to appeal to the MMO crowd... seeking mostly the "standard" male audience). The mindset and the mission behind the marketing has changed, but it's still, at the end of the day, marketing - appealing to some and turning others off, just like before. For better or worse.