That could happen. It depends on the languages you learn. If they are similar, it can happen that you get confused. I once learned Korean and Japanese and really started to get confused. Yesterday I talked with a Vietnamese girl and she asked me something in Chinese. My answer was a total mix of both language, using Chinese words with Vietnamese particles. Haha. The chance of confusion gets higher, when languages are similar, in structure and vocabulary...maybe Italian and French...German and English...Korean and Japanese etc...When you start learning, however, you will focus on your language and think about it deliberately.