However it is not possible to extend with complex TCA like inline relations, database driven select fields, FlexForm handling and similar.
Hacking it would be a recipe for problems down the road. Since the Site Configuration is accessible to admins only (and I assume you trust your admins), I guess you could use a plain text field and ask people to enter HTML.
No, i dont violate any limitation. RTE Fields in site_language worked in V11 without a problem.
I can make it work by adding a RTE field for site, then it will also work in site_language.
So this is simply a bug. It has to do with the migration to es modules. You are not able to load / modify the importmap after the initial page load. This is by design. So TYPO3 fixed the issue for TCEForms by addind the “backend.form” tag for JavaScriptModules. But there is no such tag for the Site Configuration. So this is simply a bug. Plain and simple.
Hacking it would be a recipe for problems down the road. Since the Site Configuration is accessible to admins only (and I assume you trust your admins), I guess you could use a plain text field and ask people to enter HTML.
Not true. Not everyone who is allowed to make configuration in the site config module should now everything about HTML and typolink syntax and so on. Having an RTE helps a lot.