Funnily enough, even though I always have "check spelling as you write" turned off, I can see how desirable being able to mark stretches, let alone documents, as being in a specific language would be desirable. I also know it is possible even on the Mac and using the Apple Textkit on which Scrivener is based, as Nisus Writer Pro does just that.
I am also sure KB, LAP, Tiho_D and the rest of the team would see it as desirable; but the question is more one of feasibility.
NWP is. developed by a team whose sole purpose is improving the functionality of the Apple Text Engine. KB is one person who has the development and maintenance of the whole of Scrivener—let’s not forget iOS, which is also used by many Windows users—and Scapple occupying him. Complex modification or writing replacement routines of the text engine to mark languages, and there are other aspects like tables and lists which are calling for attention, can only wait for time and energy.
For the Windows version, doing what is necessary from this point of view may be easier, but LAP and Tiho_D are having to code frameworks used by Scrivener that are available out of the box on MacOS. So for them too, time and energy are at a premium.
In addition to the question of feasibility, there is the question of compatibility. I have no objection to the Windows version having a feature which the Mac version doesn’t—we have "language focus" as opposed to just "dialogue focus" … not that I use that either—but if language-marking in Windows would cause compatibility problems, and I would think it might be even more of a problem in relation to the iOS version, then it would indeed be a bad idea.
I am no programmer, just a long-term user of Scrivener. I do any spell-checking if necessary in NWP after compile, and do use more than English; however, since the second languauge I use most is Chinese, I have a macro which can do the marking for me. Any other language, I would mark in Scrivener with a specific style and use Find & Replace in NWP to add the language marking.