"Omar is a fast translator."
Translation:عُمَر مُتَرجِم سَريع.
I think it is something in the system and how AI works on correcting answers. In Russian lessons on Duolingo I do have big problem between two similar letters (Ш and Щ). My answer would be ALL wrong if I simply typed one instead of the other even if all the grammar and structure is correct in the sentence, my answer would be wrong completely just for this little change of letters. Even some Russians in the forum told me that it's OK and such mistakes are considered typos and not linguistically serious, but the system here still not fixed in this respect.
So, I would say it is a bless that Duolingo accepted your answer with typo but just marked it for you so you can learn. The correct word is indeed مُتَرْجِم (mutarjim).