Translation:He kicked a sitting man in the leg.
It's not a fixed expression in this sentence, we also say sparka på den som ligger "kick the one who's lying [down]".
Does this mean it was intentional? If we said "sparkade på" would it be more accidental?
Yeah, "against" wouldn't make sense in English here. I an not sure why but we don't say it. I feel like (as a native English speaker, but not consulting any rules) that "against" implies a longer or more solid impact: thrown against the wall." Also you don't use "kick" with "against," usually. "In the leg" is kind of a set idiom.
I agree. I was about to make the same comment.