In general Joomla seems to lean towards being unrestrictive, not just with tags but with other areas as well. Hence for example you can set a Spanish category against a French article. I think that once you start putting restrictions like that in place, users can come up with use cases for why that restriction is wrong.
Setting language = all is also a default for new items, such as articles or categories, although you could make an argument for setting the tag language to be the same as the item language.
You can set the Global Parameter of Tags / Item Selection / Language Filter to restrict the tags which are displayed on the front end, but I don't think this applies to the back end.
As regards managing the situation, something you could probably do is in the Tags field, in the available tags which are shown, add the language in brackets after the tag. This would make the field a little like the Category field, which puts the language in brackets after the category. Administrators can then easily see what language a tag relates to.
You'd have to override the tags field by doing something similar to what is suggested in How to conditionally filter the presentation of category when creating a new category/article?.
You'd need to put in place some process around setting the language of a tag whenever it's created.
More drastic measures could involve writing a plugin which fires on onContentBeforeSave and rejects inconsistent item / tag mappings.
