Translation:He will be working here until December.
To expand on that, будет is an auxiliary verb (forming the future tense). The auxiliary verb is conjugated, and the other verb stays in infinitive. That's the same in all languages I know.
"He will WORK in December" (will BE working)
"Er wird im Dezember ARBEITEN"
"Il va TRAVAILLER en décembre"
"El voy a TRABAJAR en diciembre"
"Han skal JOBBE i desember"
This is one of the cases where I severely doubt the pedagogical thought behind some Duolingo courses.
We have just spent two checkpoints' worth of lessons with basic sentences - all right. But now, the first lesson leading up to checkpoint 3 introduces numbers, past tense, and future tense all at the same time?!