"Whom do you see?"
Translation:Wen siehst du?
Wen is nominative and mostly refers to the subject. Wen is accusative and asks about the direct object. Wem is in the dative and is used for indirect objects.
A simple rule of thumb is to assume the answer is myself. If I would reply "ich", the right question would use "wer". If I would reply "mich", the right question would use "wen". And lastly, for "mir" the question should use "wem".
I found this https://german.tolearnfree.com/free-german-lessons/free-german-exercise-109824.php though I'm not sure it explains the use if wen in this situation
I don't agree that it is "great English". Although they are often used interchangeably, that doesn't mean that it's correct; there is a distinction to be made in the usage of who and whom. Who is a subject pronoun and whom is an object pronoun. They are not the same thing.