System message in multiple languages
I have English and German installed in Windows and Firefox (Well and Russian, but Russian does not give any issues) English is set as system language in Windows. English is set as the preferred Menu and Content Language in Firefox. The menu is displayed in English as it should be.
Problem now is, some Firefox messages, i.g. the message that a website is asking me to confirm I want to leave, is in German, for apparently no reason. Even on English websites. It's not a major inconvenience, just weird.
I attempted to remove German from the alternative menu languages, for testing, but cannot. English and German both cannot be removed. A third language I add randomly can be removed.
I am aware that the locale I downloaded Firefox in cannot be removed, but I hardly downloaded Firefox in two Locales at the same time, did I? From what I can see I originally probably downloaded German Locale, since English is listed under Language Add-ons.
So what I am curious about is, how is it possible that Firefox gives me part of the menu/browser messages in English and parts of it in German? Does the English Language lack some translations? Hardly I'd guess. And why do I have two languages I cannot remove?
You can try "Clear the startup cache..." via the "Help -> More Troubleshooting Information" (about:support) page.