Default language?

I have a multilanguage site and installed some languages.
I realized, that the override.php of in the default-language (English) folder overrides also the other language files.
(my example is: I changed 'YOU_HAVE_COUNT_NEW_PRIVATE_MESSAGE' in the default-language with an override and this changing is also in for example french language, until I put 'YOU_HAVE_COUNT_NEW_PRIVATE_MESSAGE' also in the override file in the fr-fr folder.)

By this I realized, that I may have a configuration problem.

I have the default language installed, which is English. I installed already English (en-gb) beside the default language but don't know how to handle both.

I solved my override problem by transfering the override.php from the directory default-language to the directory en-gb.

But why here is the default language, for which scenario is the default language needed (in comparison to English)? What I needed to do here?

That's due to the language fallback behavior. It's supposed to ensure language strings won't be missing. This isn't intended for the overrides though. Looks like we've a bug ticket for this already below.

forge.joomlapolis.com/issues/6680

Will review implementing a fix to prevent the overrides from using the fallback behavior.