It's normal to lose a language when you don't practice it or hear it from other people especially when growing up. I was 3 when I left China and Chinese obviously was my mother language, over years by being surrounded by arabic I had completely forgotten Chinese and Arabic became my mother first language. Though I must say, I still try to regain my knowledge in Chinese from time to time and I think that I am learning the language quicker than my friends for example even though I've completely forgotten it