Function install_locales 

pub fn install_locales(default: &str, locales: &[(&str, &str)])

Register the app’s locales (see day_l10n::install) and fix the layout direction from the locale that actually resolved (docs/localization): an RTL locale (Arabic, Hebrew, …) mirrors every horizontal placement and flips the native toolkit’s direction. Direction is resolved once, before the first layout — runtime set_locale switches strings but not direction.