Function resolve_locale 

pub fn resolve_locale(
    settings_locale: &str,
    i18n_config_path: &PathBuf,
) -> String

What: Resolve the effective locale to use, following fallback chain.

Inputs:

• settings_locale: Locale from settings.conf (empty string means auto-detect)
• i18n_config_path: Path to config/i18n.yml

Output:

• Resolved locale code (e.g., “de-DE”)

Details:

• Priority: settings_locale -> system locale -> default from i18n.yml
• Applies fallback chain from i18n.yml (e.g., de-CH -> de-DE -> en-US)
• Validates locale format (basic check for valid locale code structure)