I'm natively German but nowadays I consume almost everything in English and barely ever speak German, which causes me to slowly forget my native language. I can feel my vocabulary getting more and more limited and I often have to think hard for certain words that I know immediately in English. So yeah, if you don't use a language, native or otherwise, then you'll slowly unlearn it over time. Shouldn't be too surprising, it's like this with a lot of mentally related skills, like math for example. I couldn't do most of the shit I've learnt in school at some point because I never really had a use for it and consequently forgot all about it.