My favorite quote in this vein is from the American humorist George Ade: "'Whom are you?' he said, for he had been to night school." Yes, the grammar is shaky, but "whom" usually sounds a bit formal, and many uses of "whom" seem to be going the way of the subjunctive case. (I am old enough that I was taught in grade school to say "If I be....", and I still give a little shudder at "If I was...") Language moves on.
Alejandro is correct when he said "who is playing with you", grammatically it is not proper English to end a sentence with a proposition. Although as Berniebud pointed out, the translation back into Russian (using correct English grammar) would be "кто играет с тобой". Duolingo needs work still.
Ending sentences with prepositions was in use long before any English speakers objected to it. It's called preposition stranding. See Wikipedia: