It's common to say that a machine, a program, or some kind of mechanical system is "fully operational." I've never heard the phrase used to describe a person's skills. By the way, "fluent with complete understated" definitely doesn't make any sense. Did you mean to type something like "complete understanding," or is this mistake actually in the text you're quoting from?

"Operational" or "fully operational" is used formally and sounds a bit technical. We would not use it casually or in casual conversation. It's more common to say that someone has a "good command of something". For example: "He’s studied in the US and has a good command of English." I hope this answer helps. If you are interested in increasing your English fluency, I am a professional teacher on Italki. I would be happy to work with you.

Edit: I forgot to fix my answer to mention that “fully operational control” isn’t correct but “full operational control” would be. No that’s not a very common use at all, but it is close to being phrased correctly to be accurate in that use. It’s referring to being able to operate or work with and use the language fluently and correctly. Also is that a quote from something? There are a few errors in it that don’t reflect full operational command of English.