[FR] Separate UI language from regional formatting locale

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Problem statement

We want to use InvenTree entirely in English, while using Dutch regional formatting (en-NL).

Currently UI language and regional locale are partly coupled. Some formatting uses the browser locale, while other components, such as the calendar, use the selected InvenTree language (en) as the locale. This can result in inconsistent date, number, currency and calendar formatting.

UI language and regional formatting are separate preferences. An English UI should not imply US or generic English regional formatting.

Suggested solution

Separate display language from regional / formatting locale in the user settings.

For example: Language: English (en) Regional locale: English - Netherlands (en-NL)

The language should only determine the translation catalogue. The regional locale should consistently control Intl.NumberFormat, Intl.DateTimeFormat, currency, calendars and other locale-sensitive formatting.

An "Automatic / browser default" option could be provided as fallback. Regional variants such as en-NL should reuse the normal en translations.

Describe alternatives you've considered

Adding en-NL as another UI language would solve this specific case, but incorrectly mixes translation language with regional formatting.

Relying only on the browser locale also does not work because some InvenTree components explicitly use the selected InvenTree language as their formatting locale.

Selecting Dutch provides Dutch formatting but also makes the UI (Partialy)Dutch, which is not desired.

Examples of other systems

Windows separates display language from regional format. The web platform also supports this distinction through BCP 47 locales and the JavaScript Intl APIs, e.g. en-NL.

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