Formal:
* Rydych chi'n hyderus yn yr iaith: This literally means "You are confident in the language." It implies a high level of fluency, but it's more about confidence than being completely fluent.
* Rydych chi'n fedrus yn yr iaith: This means "You are skilled in the language." It suggests a high level of proficiency, but it's not necessarily a synonym for "fluent."
Informal:
* Mae'r iaith yn dod yn naturiol i chi: This means "The language comes naturally to you." It implies fluency and ease of use.
* Rydych chi'n siarad yr iaith yn rhugl: This means "You speak the language fluently." It's the most direct translation, but it might sound a bit formal.
You could also use a more specific phrase depending on the language you are talking about. For example, if you are talking about being fluent in Welsh, you could say:
* Rydych chi'n siarad Cymraeg yn rhugl. (You speak Welsh fluently.)
The best way to express "You are fluent in" in Welsh will depend on the specific context and the level of formality you are aiming for.