Formal:
* Polyglot: This is the most common and widely recognized term for someone who speaks multiple languages. It suggests a high level of fluency in several languages.
* Multilingual: This term simply means speaking more than one language, without implying any specific level of fluency.
Informal:
* Linguist: This term is generally used for someone who studies languages professionally. However, it can also be used informally to describe someone who speaks many languages.
* Language enthusiast: This term is more casual and suggests a passion for languages, rather than just the ability to speak them.
The best term to use depends on the context and the specific nuance you want to convey.