Here are some examples:
* I'm learning French.
* She speaks fluent Spanish and English.
* Japanese is a complex language.
Exceptions:
* When referring to a language as a general concept, you don't need to capitalize it:
* She's good at learning languages.
* The study of linguistics is fascinating.
* When a language is part of a compound noun, you don't always capitalize it:
* English teacher
* French fries
* Chinese food
However, the capitalization rules for compound nouns can vary, so it's always a good idea to check a dictionary or style guide.