Multilanguage for Sign in and Registration problems

I’m working on a Site with multilanguage features. Currently only German is live and set as primary language. The problem is that the sign in as well as register page (and some other single pages) don’t render in correct language after the initial load.
So, when i open the register or sign up page as first load in an incognito browser, they will render in correct german language. If I then reload the page the site will change to the english locale and therefore everything will be in English.
Same thing happens if you open the homepage and go with an additional page load to these singlepages.

Therefore, Emails don’t get Translated versions aswell, when new users register for example.

I have checked multiple previous posts where it says this should be fixed, but it does not seem like it. For example
Default Locale of Website is set to German de_CH, Default Site Interface Locale is set to German de_DE and Default Language is set to German de_DE aswell.

I am using Concrete Version 9.1.2 (Problem was existing with previous Versions before).

Sliding in here with a similar problem concerning Languages.

Once here Set Calendar locale? and here Change backend language in v9 - #5 by donat on differnet Projects.

I only got these after upgrading past v9 (currently 9.1.3). Did you have these problems in v8.x.x as well? Seems like some locale stuff is not working correctly on v9 to me.

I remember having similar issues before with version 8.x.x as well (in other projects than the current one). But it was not important back then, because public registration was not used, and therefore only admins would need to sign in. Since there was no End-users, we did not think a lot about it.

The translation seem to work well, as long as the locale gets loaded correctly. The only thing that never translates, is the “Stay signed in for 14 days” option label.

Hmm I can see this in the de_DE PO:

 #: concrete/authentication/concrete/form.php:40
msgid "Stay signed in for %s"
msgstr "Für %s angemeldet bleiben"

And then I can see that is wrapped in a t string in the source:

<?php echo t('Stay signed in for %s', $dh->describeInterval(Config::get('concrete.session.remember_me.lifetime'))); ?>

So not sure what’s going on there - might be worth opening an issue here: