The Command spell reads:
You speak a one-word command to a creature you can see within range. The target must succeed on a Wisdom saving throw or follow the command on its next turn. The spell has no effect if the target is undead, if it doesn't understand your language, or if your command is directly harmful to it.
Emphasis mine, it mentions the target needs to understand the language not necessarily the words you use. If a caster has a much larger vocabulary than the target, does the magic aid in the comprehension and compulsion, or does the lack of comprehension block the compulsion?
Example: the cleric in question is fighting a bandit on the road through the woods and wants to make them drop their weapon and kneel on the ground and be busy down there for a few seconds could they say, “Defoliate!” and cause them to start ripping the grass out of the ground? Assuming the bandit doesn’t know the word, of course.