If I remember correctly, when you learn a new language after age 10, the brain encodes the new language in a region adjacent to the one where your mother language is encoded, which means that any localized injury on this part of the brain may impact your ability to speak just one of them.
Source: Sapolsky's lecture about language: https://www.youtube.com/watch?v=SIOQgY1tqrU&list=PL150326949...
If you want to understand better how the brain works, I really recommend watching the whole course.