User blog comment:D.Ryan/Does Rapture have an official language?/@comment-3503432-20110727141904/@comment-2250991-20110727181404

That's being too literal on the subject. Even if someone played it in their language, it'll just have different voice actors translated to their language to understand it, instead of having different characters speaking different languages for their own. In some cases, most non-English voice actors can't do an accent foreign from their own when pronouncing it in their language, so it'll be even more cryptic to tell what nationality someone like Tennenbaum or Suchong would be from listening to those voices.

Anyways, D.Ryan just meant that Rapture (as a place, not as the game) adopted an official language for everyone to communicate with each other in, which in this case, it's going to be English. The signs and music are mostly in English, and even with other people of different nationalities living there, they do speak English with an accent.