I think you can't forget the whole thing, but your brain wires itself to think faster in the language you're using the most every single day. I lived in Mexico for about 2 years (I'm Venezuelan), and even though both countries speak Spanish, I was losing my coloqualisms without noticing. Furthermore, I lived in the USA for 4 months and the switching from English to Spanish was becoming a bit of a harder transition after a while, honestly. If you live enough time in a different environment, you'll forget your own a bit!