Many answers in linguistics will be informed by cross-linguistic comparison; it is a major methodological tenet of the discipline. Given that, I think we should think of when having a separate cross-linguistic tag will be useful for the community; I propose the following guideline:
The cross-linguistic tag is for questions that specifically ask for comparisons across (as opposed to within) languages or language families.
So a question about a phenomenon in one language would not carry this tag, even though its answers could of course make reference to other languages. More subtly, a (hypothetically existing) question about the development of periphrastic verb paradigms in Romance would not bear this tag, since it is about one process in the Romance family. A question asking to compare/contrast Romance and Germanic periphrastic tenses would have the tag.
Of the questions currently tagged cross-linguistic, I think only Is the "ll" in Albanian like the sounds in other languages? and Are there languages with other spatial deixis besides "here", "there" and "over there"? meet this criterion. Other not-currently-tagged candidates for this criterion (from the first few pages of recent questions) are: