registerLocaleResource method
Adds resource as a source of text for locale (which need not have
been defined with addAvailableLocale).
Implementation
void registerLocaleResource(String locale, LocaleDataSource resource) {
(_localeResources[locale] ??= []).add(resource);
if (locale == _currentLocale || locale == _defaultLocale) {
_loadCurrentLocaleResources();
}
}