Is there a criterion for tags's specificity?
If the community wants a tag, we can have it.
That's all there is to it, really.
We are such a small community that if anyone feels like creating a new tag, you can just go ahead and add it to a question.
Anyone with at least 150 reputation can create a tag.
If there are disagreements, they can be sorted out later.
Tags can be renamed, merged, blacklisted, synonymized, and maybe there are some other actions I forget.
Bigger SE communities seem to have a policy that a new tag needs to be proposed and approved on meta, but I don't think we need that.
Bad tag ideas are so rare that we have no reason to prepare for them.
That said, it's never wrong to bring something up on meta.
Does it make sense to create romance-languages?
Feel free to go ahead and add it to any appropriate questions.
It'd be nice to have a use guidance (tag wiki excerpt) to describe the tag.
It is mostly self-explanatory, but it could mention related tags like language-evolution, medieval-latin, and tags for individual Romance languages.
It often makes sense to use a number of these tags together, and in the case of the example question you cite I'd also consider adding language-evolution.