fluent

To be fluent in something — like a language or an instrument — is to be able to use it smoothly and effortlessly.

It takes many years and lots of practice to become fluent in a second language. But when you're finally fluent, you can understand it, speak it, and write easily in it. One French class won't make you fluent — gaining fluency takes years. You can also be fluent in other things that you've practiced and mastered, like cello or computer programming. If you're doing something in a way that makes it look easy, you're fluent.