How can I mention the languages I know in my resume/CV?

It all depends who is reading your CV. If it is a linguistic or translator post then those grades would be important. In general I would suggest that an employer may be interested in whether you are fluent or have a good understanding of a foreign language. What is probably more important is whether or not you are familiar with the idioms and nuances of another language. However, on your CV it is probably better to straightforward and not to elaborate, and save any elaboration, if required, at an interview.