How, on a resume, should I describe language level which is above what's commonly described as fluent, but it's not my native language?
It's a language I speak at home, and have been doing so for 10 years.
Personally, I consider "fluent" to be the top, and anything else to be "not fluent". I would caution against being too accurate/cute.
That said, something like "expert" might be unambiguous enough or "bi-lingual in X and Y" as being a clear indication that you are equally awesome at both.