Flatpak apps lose localizations after updates because extra-languages is unset in system config

Your system information

• Steam client version: 1788652215
• SteamOS version: 3.8.16
• Opted into Steam client beta?: [No]
• Opted into SteamOS beta?: [No]
• Have you checked for updates in Settings > System?: [Yes]

When using SteamOS in a non-English language (e.g., Polish), applications installed as Flatpaks via Discover (such as Firefox, LibreOffice, KDE apps like KCalc) often launch in English or revert to English after an update.

This happens because the Flatpak system configuration has extra-languages set to unset. During updates, Flatpak purges "unused" locale extensions to save space and only retains the default en locale, effectively deleting the user's system language translation files.

Steps for reproducing this issue:

1. Set the SteamOS system language to a non-English language (e.g., Polish).
2. Switch to Desktop Mode.
3. Install a Flatpak application like Firefox.
4. Open Konsole and check flatpak config --system. Notice that extra-languages is unset (default: en).
5. Wait for an automatic Flatpak update via Discover or manually run flatpak update in Konsole.
6. Launch the application. It launches in English. Running flatpak info --show-extensions org.mozilla.firefox reveals that only the /en subpath is installed for the .Locale extension.

Expected behavior:

SteamOS should automatically sync the user's OS language choice with the Flatpak system configuration. When a specific system language is selected in Gaming Mode or Desktop Mode, SteamOS should automatically run flatpak config --system --set extra-languages in the background. This would ensure Discover always fetches and retains the correct localization packages for Flatpak apps without requiring users to use the terminal and Polkit passwords.

Workaround (for reference):

Currently, the only way for users to fix this permanently is to manually run:
flatpak config --system --set extra-languages pl followed by flatpak update (which prompts for a Polkit password).

Start by tracing how SteamOS language selection works in Gaming Mode and Desktop Mode, then inspect the system Flatpak configuration. Reproduce with flatpak config --system and flatpak info --show-extensions org.mozilla.firefox, followed by flatpak update. Done means the selected language is synchronized automatically and its locale remains installed after updates without terminal or Polkit steps.