registerLocalizedSingularNames function
Register singular forms for full-form names per locale (optional).
keys must be canonical plural names from localizedUnitName (e.g., 'kilobytes'), values singular (e.g., 'kilobyte').
Registers singular forms for full-form names per locale. Keys must be
canonical plural names returned by localizedUnitName (e.g., 'kilobytes').
Implementation
void registerLocalizedSingularNames(
String locale, Map<String, String> singular) {
final key = locale.toLowerCase();
final existing = _customLocalizedSingular.putIfAbsent(key, HashMap.new);
existing.addAll(singular);
}