The phrase "an excellent command" is correct and usable in written English.
It can be used to describe someone's strong proficiency or mastery in a particular skill or subject, often in the context of language or technical skills.
Example: "She has an excellent command of the Spanish language, allowing her to communicate fluently with native speakers."
Alternatives: "a strong grasp" or "a high level of proficiency".
Our waitress at that meal had an excellent command of English, but we had trouble communicating.
"She has an excellent command of pension law and is highly regarded among her peers," he said.
With an excellent command of English, he became a spokesperson for the dissidents and refuseniks in contacting Western correspondents to publicize their cause.
I love Lee Lorenz's cave people, stocky energetic folk, who, like most cavepeople appearing in The New Yorker, have an excellent command of the English language.
Beginning with a style drawn primarily from Parker, Rollins became a master of intelligent and provocative spontaneity that was combined with an excellent command of the tenor sax.
Most Swedes have an excellent command of English and, when on top form (which normally requires a few alcoholic drinks), can be very approachable and talkative.
He had come to the Yankees with a reputation as a strikeout king in the top Japanese league, a hard thrower with excellent command of four pitches, including a fastball that had been clocked at 98 miles per hour.
Pineda, who turns 23 on Wednesday, was a rookie last year, so he is under the Yankees' control for the next five years and is considered one of the best young arms in baseball, with a good fastball and excellent command.
Next, this process was reversed by a native English speaker with excellent command of Chinese.
The advisors are of British nationality but have an excellent spoken command of Nepali language and have a background in anthropology and sociology.
With excellent command of his fastball, and a split-finger fastball that made the Rockies hitters flail, Pelfrey never allowed Colorado to combine more than one hit in an inning.
