I have been asked “How’s your English?”
I answered: “It is improved slowly,” but I have read that I must say “It is improving slowly.”
But I don’t think the second is correct, because you improve it (by) yourself and not by another body.
You are thinking of the transitive form of the word, but improve, used intransitively (without an object), means "to increase in value, excellence, etc."
"it is improved" can be used to describe a state, but "slowly" can only describe an action. You can't refer to 'it' as both a state and an action.
To get round that, you have to use an active verb such as "improving" to match the "slowly".
A minor problem is your use of present simple passive. "It has improved" is more natural.