D&D 5E (2014) Command spell and languages

[zaratan]

To command one creature he need to understand your language. As a DM, you would allow your player use command without being proficient in some language since he only need to know 1 to 5 words in that idiom?
If there is at least someone that can teach those 5 words.

 

[TheCosmicKid]

The target needs to understand you. If you know and say the word "Sit!" in its language, then it can understand you. Nowhere in the spell description does it require that you know all the other words in their language too, and I see no reason why it should. You're not using those words -- just the one.

 

[Lehrbuch]

To command one creature he need to understand your language. As a DM, you would allow your player use command without being proficient in some language since he only need to know 1 to 5 words in that idiom?

Absolutely. The caster and target just need to have a shared understanding of the one-word command that the caster says. It's entirely possible that neither caster nor target is actually proficient in the language used.