General terms:
* Polyglot: This is the most common and formal term, referring to someone who knows and is able to use several languages.
* Multilingual: This term simply means "having or using several languages," and is a more general term than "polyglot."
* Bilingual: This term specifically refers to someone who is fluent in two languages.
* Trilingual: This term refers to someone who is fluent in three languages.
* Quadrilingual: This term refers to someone who is fluent in four languages.
Less formal terms:
* Language buff: This is a casual term for someone who is interested in and good at learning languages.
* Linguist: While this term technically refers to someone who studies languages, it can also be used informally to refer to someone who speaks multiple languages.
The best term to use depends on the context and the level of formality.