We have several different [nation x-literature] tags such as russian-literature, greek-literature, and american-literature. However, we also have country tags like united-states and japan. How should these be used differently, assuming they are not synonyms?
We don't need tags for both; however, a tag for the literature of a specific culture is useful.
Tags serve to classify questions. It is possible to be an expert in Russian literature, or Chinese literature, or the literature of any other culture. So this helps experts to quickly sort questions in their field of interest.
It also helps to classify questions where knowledge of the culture helps to understand the work.
We don't need to rigorously define what is and isn't "Russian literature" - there will always be edge cases.
As a rule of thumb, if a work is strongly informed by the author's native culture, or had a large influence in a country's cultural development, then a culture-specific tag is appropriate.
These tags can co-exist with tags for specific authors. We can have some good questions on the parallels/differences between authors from the same culture (and possibly different time periods).
The point here is to determine whether it's the nationality of the author, or the geographical location that is relevant to the question.
american-literature will be used for general question about American literature or American authors.
I'd suggest using both, but for different purposes.
Country tags (united-states, japan) can be used when the country itself is relevant to the question, perhaps because it serves as the setting for a story, or because literature-associated events occurred in that country. So use mexico for a story dealing with racial tensions in Mexico, and use yugoslovia for a question dealing with why a book was banned there.
Literature tags (mexican-literature, japanese-literature) can be used to identify the origin of the literature itself – i.e., the nationality of the author or the country it is most commonly associated with.