Formal:
* Parle très bien: This is the most direct and literal translation.
* A une très bonne maîtrise de la langue: This is a more formal way to say it, emphasizing fluency.
Informal:
* Parle super bien: This is a more casual way to say it.
* Est vraiment bon en français: This emphasizes the person's skill in French.
You can also use "parle couramment" (speaks fluently) to indicate that someone is very good at speaking French.
Here are some example sentences:
* Il parle très bien français. (He speaks French very well.)
* Elle a une très bonne maîtrise de la langue française. (She has a very good command of the French language.)
* Tu parles super bien français! (You speak French really well!)
* Il est vraiment bon en français. (He's really good at French.)