I was always taught that the proper response for the Agnus Dei in English should be, "Lamb of God, Who takes away the sin (singular) of the world..." But, now a traditional Catholic told me that sin should be plural. Can anyone tell me which is correct and why? Thanks!

Plural is correct (and it's the same in EF and OF). The Latin is "peccata", which is plural.

Thanks for the clarification! I've been saying the wrong thing all these years!

If we all just said the Agnus Dei in Latin, then we would all have been right all these years

Shouldn't it be "you take away" (rather than "who takes")? (or in old English "that takest")

At the Ecce Agnus (Behold the Lamb), He is referred to in the third person, but at the Agnus Dei (Lamb of God), He is address in the first person.

Either one is a reasonable translation.

Yeah, I thought it was "You take away"

Agnus Dei, qui tollis peccata mundi:

Lamb of God, who (relative pronoun, 2nd person singular) takes-away (verb, 2nd person singular) sins of the world...

The problem with translating it "You take away" is that the Latin is a dependent relative clause, not an independent clause; the problem with translating it "who takes away" is that the construction is impersonal in English and sounds like we're talking about the Lamb rather than to Him. The most literal way to do it is to say, "Lamb of God, you who take away the sins of the world," but that's awkward.

But when we say "have mercy on us," which is a request directed at a person, little doubt remains that 'who' is personal and short for 'you who.'

Fr. OWK is right--it is SECOND person. In English, we call it the "understood you." [YOU] Have mercy on us.

I'd be horrified if it were in the first person.

Yes, my point exactly.