Allow locale (not just language) to be set in Admin UI

[michaelbromley]

Is your feature request related to a problem? Please describe.
Currently the Admin UI supports a number of display languages specified by the 2-character ISO 639-1 code, e.g. en, de.

The app then uses this code in order to localize things like number display and localized language names, using the Intl API.

However, with a couple of exceptions (Chinese and Portuguese), this setting does not allow us to make a distinction between different locales of this language, e.g. en-US vs en-GB vs en-MY. This can lead to localization that does not reflect the expected format for people from that locale.

For example, let's say we have a shop which uses Malaysian Ringgit, and we have our language set to "English". To format a price, we will be calling the following API:

new Intl.NumberFormat("en", { style: 'currency', currency: "MYR" }).format(1299)
// -> MYR 1,299.00

The problem is, people in Malaysia never refer to their currency as "MYR", they call it "RM" (Ringgit Malaysia). So we actually should be using the "en-MY" locale:

new Intl.NumberFormat("en-MY", { style: 'currency', currency: "MYR" }).format(1299)
// -> RM 1,299.00

Describe the solution you'd like
It should be possible to set the locale (i.e. which I think can be considered synonymous with country) independently of the language.

The existing "language" dialog in the UI top right menu should be extended to also allow a "locale" selection, which is a list of countries.

Also the ui config should have an extra field for default locale.

[michaelbromley]

Reference on how the locale string is interpreted by the Intl API: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl#locale_identification_and_negotiation