That would still work since the document's language would be WXS, as determined by the file extension. Editor than asks that language to resolve a mode to a language, and if a language uses a mode itself, it just returns itself. So globally the xml mode would map to the XML language, but within the WXS language, xml would map to WXS.

I just made anguageManager.getLanguageForMode privat. It is currently only used by Language.getLanguageForMode as a fallback. Right now I don't see a use case for global mode to language mapping, especially since it's easily ambiguous.