BCP-47 tag, e.g. "en", "ar", "es" (or a region: "ar-EG").
Optionallabelhuman label in its OWN language (for a picker), e.g. "العربية".
Optionaldirwriting direction (default "ltr"; set "rtl" for ar/he/fa/ur).
OptionalnumberingIntl numbering system, e.g. "arab" for Eastern-Arabic numerals (٠١٢٣). Default "latn".
One locale in an app's set.