Something like this happened to me long ago with Spanish; my native language.
I've been speaking English since I was 5 years old. However, I stopped reading in Spanish as soon as I started learning English and would only write some sentences.
In 4th grade, while I was already good in English, I could only speak Spanish but barely read it (my parents got a phone call from the Spanish teacher and informed them that I really need to practice) and forgot how to spell many words correctly. From kindergarten to 3rd grade, I had no Spanish teacher.
Most Helpful Guy
I learned German before English, despite being born and raised in the USA. (My parents immigrated from Austria.) My parents didn't think it was important for me to know English until I was about to enter kindergarten, at which point they switched to English and thoroughly confused me. So we are in roughly the same boat?
Kids adjust quickly, so I actually became better at English, especially with exposure to other kids. To this day, I speak German at less than native level, but I can fool most native speakers for a bit. If I ever lived in a German speaking country, I'm sure it would come back rapidly. In fact, I go about every year. If I'm there for two weeks, I start dreaming in German by the end of the trip.0
Most Helpful Girl
Yes it is possible. If you don't use it often enough. I know many people that understand their native tongue but can't speak it. It's unfortunate. Which is why you must practice practice practice0