This flag reason is rarely used properly. I remember explaining to basically every single highly active user in our chat what it is about. When relying on the UI, all of them had gotten it wrong. Moderators from other sites on the network also report that it is practically never used correctly.
The reason is known: naming is hard. Here, a name was picked that can reasonably mean one of multiple things:
- the post fails at providing a usable solution to the problem
- the post fails at addressing the question
- the post fails at communicating any information between users
In the real world, all of these are a sign of a low quality. Now Stack Exchange may have some rule book which says that 1) cannot be handled by moderators and should be downvoted, 2) is Not an answer, and only 3) is Very low quality. Truth is, when an interface aligns so nicely with people's common sense, they don't check the rulebook for deviations.
My suggestion is to simply use a word which means 3, and only 3. The best one-word description I can come up with is
Unintelligible This [question|answer] has severe content or formatting problems, to the point where it cannot be understood by other users. It is unsalvageable by editing and has to be removed.
"Unintelligible" is indeed a harsh judgement of a post. But so is "Very low quality". And it is the most matter-of-fact way I can think of to describe posts like "asdfasdfas" or ones written in a foreign language.
As an alternative, we could also choose a synonym with a slightly different scope, such as incomprehensible or meaningless.
The one problem I can foresee with this is smartasses saying "this math answer is so complicated, it is unintelligible to me, I will flag". My gut feeling is that it will lead to less wrong flags than the current situation, but any improvement suggestions welcome!
Related: Rename "Very Low Quality" Flag as "Unsalvageably Bad". I agree with their assessment of "confusion", even if I don't support their exact wording proposal.
Edit removed reference to English speakers, made it a more determined "has to be removed" instead of the wishy-washy "might need to be removed" - if the flagger is not sure it has to be removed, he shouldn't be casting that flag, as suggested here.