feat($locale): Include original locale ID in $locale

Most systems use *IETF language tag* codes which are typically a combination
of the ISO 639 language code and ISO 3166-1 country code with an underscore
or hyphen delimiter. For example `en_US`, `en_AU`, etc.

Whilst the `$locale.id` comes close, the lowercase format makes it impossible
to transform to an IETF tag reliably. For example, it would be impossible
to deduce `en_Dsrt_US` from `en-dsrt-us`.

Closes #13390