T038: Adds a custom locale with ISO validation and direction specification.
Validates locale code against ISO standards, checks for duplicates, and stores direction.
Also verifies that the specified direction is consistent with the language's natural direction.
T025: Gets the complete fallback chain for a locale.
Returns a FallbackChain entity with the resolved chain, display string,
and whether a language group fallback is configured.
T029 helper: Gets the current language group fallbacks configuration.
Returns a map of locale -> fallback locale for all configured language group fallbacks.
T039: Gets the text direction for a locale.
Checks custom locale directions first, then falls back to predefined RTL language codes.
Returns 'rtl' for right-to-left languages, 'ltr' for left-to-right.
T023: Sets a language group fallback for a locale.
Validates that the fallback is in the same language group,
prevents circular references, and checks locale existence.
Also enforces FR-010: directionality constraint for fallbacks.