I understand if the existing editors can't personally proofread the submissions, but that's why peer-review exists. Or an open-source project in general where people can post corrections. Jimbo Wales didn't need to speak a hundred languages to launch Wikipedia.
Besides, projects in other languages can absolutely build upon Standard Ebooks work, but to expect Standard Ebooks itself to support other languages is just too outside the scope and expertise of the volunteers available.
As it is now, Alex is editorially responsible for all output of Standard Ebooks. Changing that would require someone with the time and experience willing to take on all the responsibilities that Alex currently has for each of those other languages.
The website and toolchain are open source, so if someone would build an international version, and do it persistently, I'm sure they would link or maybe even merge the projects a bit.