The universal translator of Faerun is called the "Common" language. No real reasoning why everyone speaks it, they just do.
So, accepting that departure from 'realism' is fine, because it's baked into the world and it's consistently always been that way. It's one of the care rules of world building.
*However*, while everyone might speak the same language for the sake of simplicity, Larian has decided that they all must speak with the same accent as well.
For me, this isn't even an argument of accuracy or realism. The audible fidelity of the game would be drastically improved by greater diversity and variation in accents. Characters like Minsc are infamous now in part because of the fantastic voice accenting and unique use of an accent (Russian). The fact that you can't close your eyes and recognize various NPCs with perhaps 2-3 exceptions is telling.
Use British accents. Use different dialects within the British accent to make unique characters. But come on. Literally look at what I posted. In 5 minutes BG 2 demonstrated more accents than BG 3 has in its entire EA. There is no excuse for that.
