Translation:Is tomorrow going to be colder than today?
The new improved accepted translation "Is tomorrow going to be colder than today?" (suggested by viordache1994 & Jacques439990) is proper English. Likewise, "Will it be colder tomorrow than today?" (M.C.Green & MicahLiuba1) and "Will tomorrow be colder than today?" (JimWeller2, also JustusRobi3 given roman2095's correction) are excellent.
It took duolingo a whole year to heed our many corrections, now if they'd only fix their persistent audio dropouts and get rid of those irritating interruption screens featuring the green owl breaking the flow — the vapid encouragements could at least be in the target language!