The phrase "to know the language" means to have the ability to understand and communicate using a specific language.
Sentences with «to know the language»
While this has made language lessons more accessible then ever, it can be difficult to assess their quality when you don't know the language in the first place. (moneysense.ca)
Such people know the language of the business and of IT. (slaw.ca)
Even though the video is only available in Japanese, it's pretty easy to understand what goes on in it even without knowing the language. (geekreply.com)