locale: Load system locale from /etc/default/locale

[Taresu]

Problem

When the system locale is configured in /etc/default/locale (e.g. LANG=pt_BR.UTF-8), PAM messages shown by slick-greeter during login (password expiry warnings, password quality checks) appear in English instead of the configured language.

The root cause is that LightDM does not load /etc/default/locale when spawning the greeter process. This is by design: in LightDM's seat.c, create_greeter_session() does not apply language configuration -> that
only happens for user sessions. As a result, the greeter inherits a minimal environment with no locale variables set.

The greeter already calls Intl.setlocale(LocaleCategory.ALL, "") at startup, but with no LANG or LC_* variables in the environment it always falls back to the C locale, and all PAM message translations are ignored.

References

• LightDM source (seat.c): https://github.com/canonical/lightdm/blob/master/src/seat.c
• Original report: Messages not honoring system locale #142 by brunosoli.
• Related discussion in Session language selector support #77.
• Also considered the solution commited by @clefebvre: 1aa6d58

Affected versions

1.0.0 through 2.2.6 (current) - all Linux Mint releases from 18.3 (Sonya) to 22.3 (Zena).

Steps to reproduce

1. Configure a non-English locale in /etc/default/locale (e.g. LANG=pt_BR.UTF-8).
2. Force a user's password to expire: sudo chage -d 0 <user>.
3. Log in as that user at the greeter -> the password change prompts appear in English instead of the configured locale.

Expected behaviour

PAM messages shown by the greeter should honour the system locale configured in /etc/default/locale.

Fix

Added load_locale() in src/slick-greeter.vala, called immediately before Intl.setlocale(). It reads /etc/default/locale, parses KEY=value lines (strips quotes, skips comments), and sets any missing locale environment variables. This is modeled on the existing /etc/default/keyboard reading pattern already used in menubar.vala (added in commit 1aa6d58).

Closes #142