var format = NumberFormat.simpleCurrency(locale: _language); <-- doesn't really what matter what language you pass
print(format.simpleCurrencySymbol("EUR"));

intl软件包可以解决这个问题

import 'package:intl/intl.dart';

void currency() {
    Locale locale = Localizations.localeOf(context);
    var format = NumberFormat.simpleCurrency(locale: locale.toString());
    print("CURRENCY SYMBOL ${format.currencySymbol}"); // $
    print("CURRENCY NAME ${format.currencyName}"); // USD
}

试试这个：

NumberFormat().simpleCurrencySymbol('USF') // $

https://api.flutter.dev/flutter/intl/NumberFormat/simpleCurrencySymbol.html