Function format_in 

pub fn format_in(
    locale_name: &str,
    key: &str,
    args: &[(String, FArg)],
) -> String

Resolve key: app bundle for the locale, then app-default, then the built-in core catalog for the locale, then core English. A pseudo-locale (en-XA, fr-XA: any locale with the -XA suffix) resolves against the locale under the suffix and accents the result. Signal args are tracked reads.