Formal:
* Polyglot: This is the most common and formal term for someone who speaks multiple languages fluently.
* Linguist: While this generally refers to someone who studies languages, it can also be used for someone highly skilled in them.
* Translator: A professional who translates languages.
* Interpreter: A professional who translates spoken languages.
Informal:
* Multi-lingual: A more general term for someone who speaks more than one language.
* Language whiz: A more casual term for someone who is very good at languages.
* Fluent speaker: This specifically refers to someone who can speak a language with fluency.
* Language buff: An enthusiast who is passionate about learning and using foreign languages.
The best word to use depends on the specific context and the level of formality you're aiming for.