Which is correct and why? Sehen is an accusative verb so I thought that "Wen" would be correct but Google Translate says that "Wer" is correct. Could someone please explain? Thanks!
DanielJerrehian's answer already pretty much covers it, but I think the root problem here is a misunderstanding of the term "accusative verb", which is not a standard grammatical term. A verb does not have a case, but it does require specific cases for the other parts of a sentence. But it's more complicated than a verb requiring a single case for everything else. Instead, the verb may require one sentence part with one case, another sentence part with another case, and possibly a third part with yet another case. All verbs require a subject, which is always in the nominative. In the sentence given, that would be wer. Nouns, or the phrases in a sentence which serve as nouns, other than the subject are called objects.
Some verbs are called "dative verbs" because in addition to the (nominative) subject, which is taken for granted, they also require an object in the dative case. This is for convenience since the verb itself isn't in the dative, it just wants something else to be in the dative. By extension you might also call a verb accusative if it requires an accusative object, but these are already called transitive verbs, so having a second name for them can be confusing. A verb might require both an accusative object and a dative object; the grammatical term for this is ditransitive. Unfortunately, while traditional grammar terminology supports subject only (intransitive), subject+accusative object (transitive), and subject+accusative object+dative object (ditransitive), there is no such term for subject+dative object. Hence the need for the rather ad hoc and confusing term "dative verb". It would be difficult to consistently generalize this terminology to more verbs because there are many variations: verbs that require an impersonal object, a reflexive object, an adjective, a specific type of subclause, or some combination of the above.
The correct answer is "Wer hat dich gesehen?"
The verb sehen is accusative (the grammatical case where only the masculine article changes), meaning the sentence contains a subject and a direct object. In the sentence Wer hat dich gesehen?, the the subject is Wer and the direct object is dich.
The other sentence Wen hat dich gesehen? is incorrect because you are referring to two accusative pronouns without a subject. A way to think about it would be like this:
Wer = Nomativ = Subject
Du = Nomativ = Subject
Wen = Wer in the Akkusativ Case = Direct object
Dich = Du in the Akkusativ Case = Direct object
An example of the sentence with Wen would be: Wen hast du gesehen? = Who did you see?