He was born in Romania and moved to the United States as a child. He has stated in interviews that he still speaks Romanian fluently, and even uses it on occasion for roles.
While it's not confirmed, there's speculation that he may also have some knowledge of German. This is due to his role as Bucky Barnes in the Marvel Cinematic Universe, where his character speaks German in several scenes. However, it's unclear if Sebastian learned German for the role or if the dialogue was dubbed later.