Basically, you have two options: either improve your languages and bring it up to the level that is required by CEMS, or learn a new language at beginner’s level.
If you go for the second option, it means that you drop your original third language and replace it by the new language. If you consider doing this, choose a language that is challenging and can help you in your future career. For example, for a Western student it is more challenging and more rewarding to learn Eastern languages such as Mandarin or Japanese than another related Western language.