Formal:
* Bilingual: This is the most common term and refers to someone who speaks two languages fluently.
* Multilingual: This term applies to someone who speaks more than two languages.
* Polyglot: This word is used for someone who speaks many languages, often implying a high level of fluency.
Informal:
* Fluent in multiple languages: A more general way to describe someone who speaks more than one language.
* Language buff: This informal term can be used for someone who enjoys learning and speaking languages.
Other:
* Native speaker: This refers to someone who learned a language from birth.
* Second language speaker: This refers to someone who learned a language after their native language.
Ultimately, the best term depends on the specific situation and the level of formality you want to convey.