I suspect the issue may be due to the fact that when creating subpages of a module or the /doc subpage, the Translate extension (or MediaWiki core) defaults to the content model of the module.

It’s set by Scribunto at rELUA includes/common/Hooks.php:256-273 (at 3b2d40f28de5), not by core or Translate. This code already exempts TemplateStyles (T200914), so probably documentation translations can be exempted, too.

Change 616281 had a related patch set uploaded (by Pppery; owner: Pppery):
[mediawiki/extensions/Translate@master] Force translation pages to have same content model as source page

https://gerrit.wikimedia.org/r/616281

The patch seems fine to me. One other idea I had was if we made Scribunto::isDocPage true for /doc and any of its subpages, which would include translation pages. Is there any use case for having Lua pages under Module:Foo/doc/... ? Or any reason we shouldn't do that and only do it for i18n pages?

My patch to the translate extension would be a good idea regardless, as it fixes the case where a non-doc page is manually given the wikitext content model (through an explicit content model change or a move), and then marked for translation.

Change 616281 merged by jenkins-bot:
[mediawiki/extensions/Translate@master] Force translation pages to have same content model as source page

https://gerrit.wikimedia.org/r/616281