Type Alias LocaleSetter 

pub type LocaleSetter = Arc<dyn Fn(&str) -> Option<LocaleChange> + Send + Sync>;

Switches one app to another locale, answering with what changed, or None when the tag is not a locale at all and nothing moved.

Naming a locale the app has no catalogue for is not a failure: every message falls back, which is the rule startup already follows.

Aliased Type

pub struct LocaleSetter { /* private fields */ }