What were you trying to do that didn't work?

• In attempt to get ahead of rhel-10 Firefox flatpak automation, I have setup flatpak from rhel-9 on rhel-10 machine. As I understand it, there should not be many changes between rhel-9 and rhel-10 flatpak so the errors should be the same once the rhel-10 flatpak is released.
• Changing locale with flatpak command with produce an error message:

$ flatpak run --env=LC_ALL=ach_UG.utf-8 org.mozilla.Firefox
/usr/bin/bash: warning: setlocale: LC_ALL: cannot change locale (ach_UG.utf-8)
/usr/bin/sh: warning: setlocale: LC_ALL: cannot change locale (ach_UG.utf-8)
(firefox:2): Gtk-WARNING **: 19:00:37.916: Locale not supported by C library.
    Using the fallback 'C' locale.

• The very same message is produced also when I do not use the --env:

$ LC_ALL=ach_UG.utf-8 flatpak run org.mozilla.Firefox

• The issue is: the locale will get changed, the Firefox is translated correctly from what I could see. Therefore the error is confusing as to why it is there.
• It does not matter what locale I use, all I tried worked, some produced the error and some did not.

Please provide the package NVR for which bug is seen:

How reproducible:

• 100%

Steps to reproduce

1. Start a Firefox flatpak with changed locale from terminal.
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Expected results

• No error message

Actual results

• Error message