Formal:
* Polyglot: This is the most common and formal term. It literally means "speaking many tongues."
* Multilingual: This term is more general and simply means "speaking multiple languages."
Informal:
* Language whiz: This is a more casual way to describe someone who is very good at languages.
* Linguistic virtuoso: This term implies a high level of skill and mastery in languages.
* Language buff: This is a more informal way to describe someone who is enthusiastic about languages.
The best term to use depends on the context and the level of formality you want to convey.