General fluency:
* Articulate
* Smooth
* Effortless
* Easy
* Glib
* Eloquent
* Silver-tongued
* Well-spoken
Fluency in a language:
* Proficient
* Competent
* Skilled
* Native-like
* Bilingual
* Multilingual
Other:
* Fluid
* Graceful
* Seamless
* Natural
* Unimpeded
The best synonym will depend on the specific situation and what you want to emphasize.