It's possible to add, map or blacklist locales for specific projects. We don't generally add country-specific locales like es-MX or it-IT.
Translate provides quality assurance features including a translation review. It was initially restricted to a select group of editors but in the end there was no use for the distinction and now all translators are automatically allowed to review each other's translations. Usually, any disagreement is quickly resolved simply by explaining your reasons: on a wiki it's easy to determine who did what and to contact them.
So our suggestion is to give it a try and avoid overthinking things beforehand. Solutions can be found if any real problem arises, but for now an approval system is a solution in search of a problem.
Nemo said it well. I would like to add that there are some cleanups on going which language tags are enabled generally and which are not.
Thanks! Can you link to the page where I can blacklist a locale for a specific project? I help manage iNaturalist.
Regarding approval, the problem is that we have country-specific portals into our platform with groups that manage them, and sometimes those groups want approval control over translations to ensure their users see consistent use of language on their portals, and they get frustrated when they translate a string one way and some other translatewiki user comes along and changes it. They don't want to have to engage in the kind of constant negotiation process you're suggesting, they just want to translate it once and lock it down. For example, I think our Mexican partners have had to re-translate switches from "Tú" to "Usted" numerous times and they're tired of it. From what you're saying, it sounds like locking things down is impossible on translatewiki, which is fine, but I just wanted to describe the problem such a feature would solve. Whether or not it is a problem is pretty subjective, but from the perspective of these particular people, it is a problem.