Translations on Multisite with one domain per language

I have trouble with using “Translate Site Interface”.
On my website, I have multiple domains added as sites with the “Multiple Site Hosting”.
Each of these sites has one language added in “Multilingual Setup”.

Here’s my Problem:
If the “Site interface source locale” in the Multilingual Setup is the same as the “Default Locale”, I can’t edit the Site Interface translation file.
If the “Site interface source locale” is undefined (Unknown/Mixed Language), I am able to edit the translations, but I can’t export/save the translation file.
If I set the “Site interface source locale” (e.g. en_GB) to something else than the “Default Locale” (e.g. en_US),I can edit and save the translations. But other problems appear with functions, that rely on the sites locale. The site will have the wrong locale.

I always set “Site interface source locale” to en_US, since it makes sense (everything is written in English, so no need to translate it).
It shouldn’t matter what “Default Locale” locale you set.

Not sure what do you mean by “But other problems appear with functions, that rely on the sites locale”?

I need to translate en_US to make the hard-coded translations (t()-function in theme-templates) on the frontend accessable to the customer.
The same is needed for the translations on the other domains (.de => de_DE, .fr => fr_FR, etc.)

So, you are not able to translate strings in “Translate Site interface” or translation doesn’t work on pages?

It doesn’t work in “Translate Site interface”, when the “Site interface source locale” is the same as the “Default Locale”

Doesn’t work means something is bugged or is it just displaying “Translation s not available” (or something like that). Why don’t you just change it to en_US?

This is my configuration:
I have 2 Sites: and

Each Site has one language:
de_DE for
en_US for

On “Translate Site Interface” I’m not able to translate.

If I configure another language for a site, I’m able to translate that language. But that’s not what I want. I want to translate the default language for each site and I don’t need another language on the site.