Careful users of the language would argue that one can have command over people, but not over language.
You can for example say: “He has total command over his men. They will do whatever he wants them to”. But when it comes to language, they would prefer “of ” to “over”.
Examples: “We are looking for someone who has good command of English”. Many children in the south have little or no command of Hindi. I am told that Prakasham has good command of French.
There is a tendency these days however, even among some native speakers, to say “have good command over English”. The purists may not like this, but as the Americans would put it, that’s the way the cookie crumbles. To be on the safe side, avoid saying “command over English”.