I think the need for editing is obvious, and if someone is able to 'help' the question without changing the intent of its author they will. In some cases, that is not always easy.
I would caution against this tag, as someone could ignore it, thus shutting out any question that might not be worded perfectly. And while the 'rules' may say that only the OP can apply that tag, in practice, anyone can and will apply it. Similar to 'homework'. Additionally, as ccomet points out in comments, tags should describe the content and topic of a question, not the quality.
I've fixed dozens of similar questions, and even helped to translate others that were posted entirely in Spanish just by normal browsing of my favorite tags.
It would be 'neat' to have a way for editors to get a bird's eye view of posts that need work, however I'd be more in favor of an additional flag mechanism (e.g. "Needs Editing") rather than a tag.
Unfortunately, that would restrict the view to only 10k+ users or moderators, however 10K + users already can see a list of questions that need help, just by looking at the top down-voted questions. In many situations, a question made its way to that list (at least partly) due to a language barrier.