All of the work in polishing an epub is the chore of transcription and then nitty gritty details like correctly tagging things like roman numerals and embedded poems and following some sort of standardization guide.
The thing that Standard Ebooks does, aside from being decisive over its standards, is then require the epub to go through a review process by its creator who is a domain expert in the craft. This expert bottleneck is a big reason why SE book quality is so reliable. Accepting other languages drastically changes and perhaps even relaxes this bottleneck and changes the whole organization.
In another comment, I think you suggest that it might be time to try to persuade him to accept non-English books:
> Agreed, it's worth bringing up again on the mailing list; will do this week and post here a link to my message.
But it's not really up to persuasion. Because it takes more than an idea to expand the accepted languages. It requires at least one reliable expert in that language who can stand up a completely new set of tools and standards and then steward that project to fruition. And that's such a big undertaking that it's really a whole new project, not just yet another egg under SE's wings, but a whole new chicken coop.
Suggesting that Standard Ebooks move to support other languages is 0.0000001% of the work towards that goal. People do that all the time, then claim "okay, I'll fork the project", and then fizzle out.
Another example is that whole swathes of code in https://github.com/standardebooks/tools, SE's core workflow, become useless once you're targeting something other than English. By browsing their style guide and that repo, you'll realize that SE's value really is its focus on English. It's more of a suite of English tools than it is a epub editing kit, as the latter is the easy part.