This is a mistake. The list should be the set of languages both people already know. Letting syntax learning be a barrier to problem solving will not be fun and it will create an initial imbalance in the pairing.

You will eventually want to move to an ELO-ranking like system to make sure the pairs are roughly balanced. Otherwise, there's no way it would be fun for an expert-level C developer to pair with a newbie JavaScript programmer and have to spend the whole time explaining what pointers are, etc.