"Who are their costumers?"
Translation:Siapa konsumen mereka?
Halo teman! Your sentence, "Who are their costumers?", it is already suggested by Duolingo.
Selamat belajar! :)
I checked my dictionaries and it should be customers. One of my dictionaries autocorrected costumer to customer so I suspect their dictionary did the sale. A different dictionary gave both (customer and costumer) as pelanggan ... but would only translate pelanggan as customer. Since most translation apps use the same database (alpha-tru) I suspect the error comes from there.