It's hard to tell without seeing your PB. Dustin brought up some great points.
I'm wondering if it has to be twins. Could it be cousins, which would make it easier to understand why they speak two different languages?
The ending needs to be satisfying. There are great books like WHEN A DRAGON MOVES IN where readers can decide if it really happened or it was just in the boy's imagination, but readers shouldn't leave with an unsettled feeling. The best endings are surprising, yet when you look back at everything, make total sense--and make readers want to dive into the book again and again.
Have you run it through your critique group and/or the critique section of our board?