registerLocalizedUnitNames function
Registers custom localized full names for a locale. Keys must be
canonical symbols (e.g., KB, MiB, kb).
Implementation
void registerLocalizedUnitNames(String locale, Map<String, String> names) {
final key = locale.toLowerCase();
final existing = _customLocalizedUnits.putIfAbsent(key, HashMap.new);
existing.addAll(names);
}