@KevinH: I would like to return to the issue SIGIL_ONLY_USE_LANGCODES described in post
#120.
The current implementation has a side effect, which is that the UI language list is missing information, because when we limit the list of languages, it is done globally – for the entire Sigil.
Screenshot:
So I suggest partially incorporating Doitsu's latest suggestion from post
#6 and meeting halfway: let's add at least those languages that have translations for the Sigil interface to the list of languages declared in the environment variable.
I have a working solution that you can certainly optimize.
The result is an expanded list of languages, which of course also appears in the metadata editing window:
I already have an updated EnvVarEditor plugin that supports the new environment variable, but I want to test it a little more before the official release. The language validation function will not allow duplicates or incorrectly written language codes. IMVHO – it works great

and really makes my life easier.