Well, the question is in the title. I should translate one phrase and I don't know the right answer. What is his mark? or What mark does he have? Which one is correct? What the difference between them?
None of them are wrong; in fact, they're used interchangeably in school settings. However, there is a difference between them.
What is his mark in Science? His mark is an 84 [percent].
The mark has a value. This question is simply asking what that value is. His mark is the subject.
What mark does he have in Science? He has an 84 [percent].
He has a certain mark. This question demands the same answer as the previous one, but it's phrased in a less direct fashion, instead asking for specifications on something he has. He is the subject. In conversations between students, this is probably the most common.
An example brought up in the comments was
What mark did he get? He got an 84 [percent].
This is different from the other two because it implies that the mark was given at an earlier time, while the others ask for the mark at the current time.