Coworker is an appropriate translation.

同僚

All those '동' words are cool and useful, but authors of the course left out the two best ones... Well, that means it's time for me to do some... minor upgrade :)

So there are those two words, which, for... some reason, are quite unpopular in South Korea, but their popularity flourishes in North Korea, so they are more than worth mentioning: 동무 and 동지

Both are usually translated into English using a noble word 'comrade'. However, they are not interchangable and it is crucial to use the right one in the right circumstances.

So first of them, '동무' is a word of native Korean origin (unlike virtually any other '동'-word) and basically means a close friend or someone one grew up together with. Despite that, it is used as a standard way any two people on the same level (or a person on higher level and one on lower level, like an elder to a child) refer to each other in North Korea.

우리 동무들 뭘 하는가? - what are our friends (comrades) doing?

The second one, '동지' is of chinese origin (同 - same, 志 - will) and means a person who shares ideological values with us and takes part in common struggle. This is much more closely related to the communist 'comrade'. Yet still, it is a a word used on daily basis, mostly when refering to a person we are NOT close to and we are NOT on the same level. It is generally more formal.

동지 is an almost direct equivalent to '선생님' used in the South. The biggest difference is that 선생님 is usually used alone (선생님, 여기 오세요 - sir, please, come here) while some sort of title is often attached to 동지 (판매원동지, 여기 오십시오 - comrade shopkeeper, please come here)