Add locale to date_utils

lib/date_utils.dart

@@ -15,16 +15,47 @@ class Utils {
   static String fullDayFormat(DateTime d) => _fullDayFormat.format(d);
   static String apiDayFormat(DateTime d) => _apiDayFormat.format(d);
 
-  static const List<String> weekdays = const [
-    "Sun",
-    "Mon",
-    "Tue",
-    "Wed",
-    "Thu",
-    "Fri",
-    "Sat"
+  static String _locale = 'en';
+
+  static List<String> get weekdays => _getWeekdays();
+
+  static const List<String> _locales = const [
+    "en",
+    "ja",
+    "zh",
+    "ko",
+    "de",
+    "fr"
   ];
 
+  static const Map<String, List<String>> weekdaysByLocale = const {
+    "en": const ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"],
+    "ja": const ["日", "月", "火", "水", "木", "金", "土"],
+    "zh": const ["日", "一", "二", "三", "四", "五", "六"],
+    "ko": const ["일", "월", "화", "수", "목", "금", "토"],
+    "de": const ["So", "Mo", "Di", "Mi", "Do", "Fr", "Sa"],
+    "fr": const ["dim", "lun", "mar", "mer", "jeu", "vev", "sam"],
+  };
+
+  /// Set Locale
+  static String setLocale(String newLocale) {
+    if (_locales.contains(newLocale)) {
+      _locale = newLocale;
+    } else {
+      _locale = 'en';
+    }
+
+    return _locale;
+  }
+
+  /// Get Locale
+  static String get locale => _locale ?? 'en';
+
+  /// The list of weekdays
+  static List<String> _getWeekdays() {
+    return weekdaysByLocale[locale];
+  }
+
   /// The list of days in a given month
   static List<DateTime> daysInMonth(DateTime month) {
     var first = firstDayOfMonth(month);

test/date_utils_test.dart

@@ -138,4 +138,40 @@ void main() {
       });
     }
   });
+
+  group('Locale for weekdays', () {
+    setUpAll(() => Utils.setLocale('en'));
+
+    test('locale in Default', () {
+      expect(Utils.locale, 'en');
+    });
+    test('locale in English', () {
+      Utils.setLocale('en');
+      expect(Utils.locale, 'en');
+    });
+    test('locale in English and Weekdays in English', () {
+      expect(Utils.weekdays, const ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"]);
+    });
+
+    test('locale in Korean', () {
+      Utils.setLocale('ko');
+      expect(Utils.locale, 'ko');
+    });
+
+    test('locale in English (set) and Weekdays in English', () {
+      Utils.setLocale('en');
+      expect(Utils.weekdays, const ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"]);
+    });
+
+    test('locale in Korean and Weekdays in Korean', () {
+      Utils.setLocale('ko');
+      expect(Utils.weekdays, const [ "일","월","화","수","목","금","토"]);
+    });
+
+    test('illegal locale is fallback to English', () {
+      Utils.setLocale('ILLEGAL LOCALE');
+      expect(Utils.locale, 'en');
+      expect(Utils.weekdays, const ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"]);
+    });
+  });
 }