I think we never could forget our mother tongue, because we've been engage with it since a kid. It has adhere to our brain like the sticky super glue.

As time goes by, we learn other language. The hardest thing to learn other language is how to make our brain think as that new language we learn not to think in our mother tongue.

I feel that with every language I'm improving, I'm making my other languages a bit worse. :) Spanish has caused a LOT of damage to my French. I also know that my Dutch is less good than it was 25 years ago, when I left my country. My sister says that I now say a lot of things in a very American way. It's hard to completely forget your own language though, as long as you keep actively using it.