Not sure if it is all that common; but, it happened with a previous game and so I re-nstalled a new game and even to a different system. I played up to the point where it appears, on a vanilla game with no mods added, and the same results. Not sure how it's happening or why; but, it's more of a nuicense than a real issue. It first appeared when you started the quest with the dog Barbas. Later, it infiltrates other things like some trader shops showing up in German, etc.
My game is now modded and so I've added an Asian follower that speaks, Japanese, (I think, as I can't understand her either). This means that I now have a game that is basically multi-lingual, except that I even have a hard time understanding English at times. I'm in the US btw.
I really don't need it fixed. It's more of a curiosity than anything. I was wondering if this has happened to anyone else.
Needless to say, I am having a blast with this game. What? "I need to take my Ingel shtikerslazher and go to the Planken shlicker, located in the Ober domensie"? Not a true translation of course. Just my attempt at being funny. Doing the quest with Barbas; however, might have been rather hard if not for the quest markers showing where to go.
Anyway, I now have more than 600 hours in this game and I'm loving evry minute. Present level is in the mid 20's.