Function speak_window_language 

pub fn speak_window_language(catalogues: &[(String, String)]) -> String

Install a package’s catalogues ((locale, Fluent text)) and make this window speak one language: the person’s when the package ships it, the package’s source language otherwise. The toolkit’s own words and the platform’s formats follow the same language, so a window of an app written in one language reads in that language throughout. A host that draws one window on a thread calls this when the package loads; its language stays as the window’s.