[Panel] PHP locale string is incorrect

[ghost]

Describe the bug
Just created my first (default) language in the Panel: English.

PHP locale string is set to en_US by default, so I changed it to en_GB. The new locale string is set correctly in the file site/languages/en.php, but after reloading the Panel, PHP locale string is reset to en_US (only there, not in the file).

To Reproduce
Steps to reproduce the behavior:

1. Enable languages in site/config/config.php
2. Go to the Panel > Settings
3. Create the first language: English (all fields automatically fill in corresponding values, including en_US)
4. Change en_US to en_GB and add the language
5. Edit the newly created language, it says en_US now.
6. Go to site/languages/en.php, the value LC_ALL is set to en_GB.

Expected behavior
The Panel should display the correct locale string.

Screenshots

Kirby Version
3.5.1

Console output
No output.

Desktop

• OS: macOS
• Browser: Safari
• Version: 14.0

[bastianallgeier]

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