The development of the first language is one of the aims of CLIL. When children are managing more than one language, different stages of mixing and interfering are usual as a part of the whole process. There is no evidence showing that if learning in the additional language will hurt the first language though. Furthermore, if we make children capable of thinking about any concept in different languages we will help enrich their way of thinking and connecting ideas.
The more languages we learn, the more open-minded we will be!