I might translate this sentence as: Whom (who) do you see? rather than the progressive "are you seeing". The translation given- whom are you seeing?- is grammatically correct, since whom, and not who, is used whenever the person in question is the direct object (not just the object of a preposition as in "to whom, by whom, etc). It does sound strange to my American English brain, because we tend to only use who in this instance. In any case, if it were talking about dating someone, the verb would be salir, not ver (Con quién estás saliendo?).

Actually 'whom' is technically supposed to be used instead of 'who' as an object in all cases, i.e. verbs and prepositions. "Whom are you dating?" etc. is correct because 'whom' is the object of the verb 'dating,' despite its appearance at the beginning of the sentence.

I thought it meant "Who are you watching?" which is very different from your suggested meaning. Another thing about "whom".. I once taught reading to 12 years in a school. All of them when they came across "whom" didn't recognize it. It is on its way out of at least spoken English.

Whom is literally the object form of the subject Who. It's not grammatically erroneous to use it here at all.

I disagree that it is not natural. Uncommon, yes, but there are many of us who use this kind of construction with nary a second thought.

Actually, "whom are you seeing" is grammatically correct in English and would mean the same thing. "Whom" is just falling out of anything but formal usage.

It's not unnatural, just archaic, or, at best, extremely formal. 'Whom' refers to direct objects when used without a preposition, so it's use here is correct.

In the modern vernacular, 'whom' is practically dead, however, though it still hangs around in zombie form with prepositions in some varieties of formal English.

I put that and was rejected 7/4/15

DUO did accept the "whom are you looking at" version which is annoying since it is more correct and sounds better to use, "at whom are you looking," which they did not accept. Oh well....

FredHasson: If you were in the pub with the guys, and one of them was eyeing up a pretty girl, would you say to him "At whom are you looking?" I don't think so!

I agree, 'At whom ..' is proper English and a direct translation of 'A quien...; DL just missed this one.

That's what I put and it answers all the above problems.

Certain pronouns, like quién, require the personal "a". http://spanish.about.com/cs/grammar/a/personal_a.htm

Yes, you'd have to use a different tense ("vas a ver" would probably be fine) because when you use the present progressive (estás viendo) you're referring to an action that's happening at that very moment.

In my opinion it's a mistake. Report it. I did.

A gerund is a verb ending in -ing when used as a noun. "Shopping" is a gerund when you say "I like shopping", but NOT when you say "I am shopping for groceries."

Google Translate is crowd sourced definitions, meaning that the translations are user-defined through majority votes.

I consider it a well-intentioned but overly optimistic failed experiment. As you can tell from the discussions here on duo, the gestalt mind understands neither English or Spanish very well, yet has no shortage of opinions about what is correct. I expect the majority of Google Translate users is drawn from this same crowd of highly opinionated beginners, since anyone with any actual ability with a language quickly realizes how bad Google Translate is, especially at basic translations like this.