I think Finny might be originally German or at least the lab where he was experimented on was in Germany.
My basis for thinking this is when they show the flashbacks of his past, everybody (including him) is talking with square speech bubbles. These square speech bubbles are identical to the ones used before when they were in Germany to show when people were speaking German. This would also explain why Finny knows how to speak German.
I'm curious if anyone agrees with this or had the same thought.
I just finished the Emreld witch ark and saw in Finnys past that all the boxes are squares showing that they speaking German. Maby the lab is in Germany but by the time Sebastian teaches Finny to read he can speak English.