Sharing a language vs. sharing a vocabulary vis a vis the Command spell

Question

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.

Answer 1

There are no rules or mechanisms for vocabulary

A language, from the point of view of the rules, can be known (in which case you understand, can read, and write it), or not known (and thus not understood). As there are no rules to cover, it falls entirely to your DM.

If you're worried about it, you should ask them how to handle it. If you're going to use commands outside the listed ones, and especially odd words, you should probably do that anyway. If nothing else to ensure that your DM knows what the word means. Making sure they understand your language is kinda important :)