feat(i18n): add nine locales and grammatical form support

I18N.md

@@ -1,19 +1,25 @@
 # I18n
 
 - [Introduction](#introduction)
+  - [Month forms in dates](#month-forms-in-dates)
+  - [Relative time forms](#relative-time-forms)
 - [How to use](#how-to-use)
+- [Locale data conventions](#locale-data-conventions)
 
 ## Introduction
 
 Now the i18n system of day.dart contains the following keys:
 
 - Months
 - MonthsShort
+- MonthsFormat (optional)
+- MonthsShortFormat (optional)
 - Weekdays
 - WeekdaysShort
 - WeekdaysMin
 - AM
 - PM
+- RelativeTime
 
 For example, let’s take a look at [i18n/en.dart](https://github.com/dayjs/day.dart/blob/master/lib/i18n/en.dart):
 
@@ -75,6 +81,33 @@ But in different languages, the **Short** of months and weekdays isn't fully the
 
 You can view other locale files like [i18n/zh_cn.dart](https://github.com/dayjs/day.dart/blob/master/lib/i18n/zh_cn.dart) for more details.
 
+### Month forms in dates
+
+Locales can provide optional `MonthsFormat` and `MonthsShortFormat` maps,
+indexed from 1 to 12. These forms are used for `MMMM` and `MMM` when the
+month token immediately follows `D` or `DD`, separated by whitespace or
+bracketed literal text. Otherwise, `Months` and `MonthsShort` are used.
+Locales without these optional maps keep their existing behavior.
+
+These two optional maps are currently used only for Russian month forms.
+
+For example, Russian `MMMM` displays `апрель`, while `D MMMM YYYY` displays
+`30 апреля 2019`. `D [day] MMMM` also uses the date form; `[D] MMMM` does not,
+because the day token is escaped.
+
+### Relative time forms
+
+With the relative time plugin, unit entries in `RelativeTime` can be strings
+or callbacks with the signature `String Function(int count, bool withoutSuffix)`.
+The callback returns a template; `%d` is replaced with the absolute count.
+`withoutSuffix` is true when the optional boolean argument to `from`, `to`,
+`fromNow`, or `toNow` is true. The `future` and `past` entries remain strings
+containing `%s`.
+
+For example, Russian uses `минута` without a suffix, but `минуту` in
+`через минуту` and `минуту назад`. German similarly uses `eine Minute` on
+its own and `einer Minute` in `in einer Minute` or `vor einer Minute`.
+
 ## How To Use
 
 For global usage:
@@ -103,3 +136,13 @@ void main() {
 ```
 
 The local usage's priority is higher than the global usage.
+
+## Locale data conventions
+
+Locale data follows [Unicode CLDR](https://cldr.unicode.org/translation/date-time/date-time-names)
+conventions for each language and region, including spelling, capitalization,
+abbreviation punctuation, and spacing.
+
+Relative times use natural expressions such as "a few seconds" and "a minute".
+Existing localized expressions are retained where CLDR does not provide a
+translation.

lib/i18n/de.dart

@@ -0,0 +1,81 @@
+final Map<String, dynamic> locale = {
+  'Name': 'de',
+  'Months': {
+    1: 'Januar',
+    2: 'Februar',
+    3: 'März',
+    4: 'April',
+    5: 'Mai',
+    6: 'Juni',
+    7: 'Juli',
+    8: 'August',
+    9: 'September',
+    10: 'Oktober',
+    11: 'November',
+    12: 'Dezember'
+  },
+  'MonthsShort': {
+    1: 'Jan.',
+    2: 'Feb.',
+    3: 'März',
+    4: 'Apr.',
+    5: 'Mai',
+    6: 'Juni',
+    7: 'Juli',
+    8: 'Aug.',
+    9: 'Sept.',
+    10: 'Okt.',
+    11: 'Nov.',
+    12: 'Dez.'
+  },
+  'Weekdays': {
+    1: 'Montag',
+    2: 'Dienstag',
+    3: 'Mittwoch',
+    4: 'Donnerstag',
+    5: 'Freitag',
+    6: 'Samstag',
+    7: 'Sonntag'
+  },
+  'WeekdaysShort': {
+    1: 'Mo.',
+    2: 'Di.',
+    3: 'Mi.',
+    4: 'Do.',
+    5: 'Fr.',
+    6: 'Sa.',
+    7: 'So.'
+  },
+  'WeekdaysMin': {
+    1: 'Mo.',
+    2: 'Di.',
+    3: 'Mi.',
+    4: 'Do.',
+    5: 'Fr.',
+    6: 'Sa.',
+    7: 'So.'
+  },
+  'AM': 'AM',
+  'PM': 'PM',
+  'RelativeTime': {
+    'future': 'in %s',
+    'past': 'vor %s',
+    's': 'ein paar Sekunden',
+    'm': (int n, bool withoutSuffix) =>
+        withoutSuffix ? 'eine Minute' : 'einer Minute',
+    'mm': '%d Minuten',
+    'h': (int n, bool withoutSuffix) =>
+        withoutSuffix ? 'eine Stunde' : 'einer Stunde',
+    'hh': '%d Stunden',
+    'd': (int n, bool withoutSuffix) => withoutSuffix ? 'ein Tag' : 'einem Tag',
+    'dd': (int n, bool withoutSuffix) => withoutSuffix ? '%d Tage' : '%d Tagen',
+    'M': (int n, bool withoutSuffix) =>
+        withoutSuffix ? 'ein Monat' : 'einem Monat',
+    'MM': (int n, bool withoutSuffix) =>
+        withoutSuffix ? '%d Monate' : '%d Monaten',
+    'y': (int n, bool withoutSuffix) =>
+        withoutSuffix ? 'ein Jahr' : 'einem Jahr',
+    'yy': (int n, bool withoutSuffix) =>
+        withoutSuffix ? '%d Jahre' : '%d Jahren'
+  }
+};

lib/i18n/es.dart

@@ -0,0 +1,75 @@
+final Map<String, dynamic> locale = {
+  'Name': 'es',
+  'Months': {
+    1: 'enero',
+    2: 'febrero',
+    3: 'marzo',
+    4: 'abril',
+    5: 'mayo',
+    6: 'junio',
+    7: 'julio',
+    8: 'agosto',
+    9: 'septiembre',
+    10: 'octubre',
+    11: 'noviembre',
+    12: 'diciembre'
+  },
+  'MonthsShort': {
+    1: 'ene',
+    2: 'feb',
+    3: 'mar',
+    4: 'abr',
+    5: 'may',
+    6: 'jun',
+    7: 'jul',
+    8: 'ago',
+    9: 'sept',
+    10: 'oct',
+    11: 'nov',
+    12: 'dic'
+  },
+  'Weekdays': {
+    1: 'lunes',
+    2: 'martes',
+    3: 'miércoles',
+    4: 'jueves',
+    5: 'viernes',
+    6: 'sábado',
+    7: 'domingo'
+  },
+  'WeekdaysShort': {
+    1: 'lun',
+    2: 'mar',
+    3: 'mié',
+    4: 'jue',
+    5: 'vie',
+    6: 'sáb',
+    7: 'dom'
+  },
+  'WeekdaysMin': {
+    1: 'LU',
+    2: 'MA',
+    3: 'MI',
+    4: 'JU',
+    5: 'VI',
+    6: 'SA',
+    7: 'DO'
+  },
+  'AM': 'a.\u202fm.',
+  'PM': 'p.\u202fm.',
+  'RelativeTime': {
+    'future': 'dentro de %s',
+    'past': 'hace %s',
+    's': 'unos segundos',
+    'm': 'un minuto',
+    'mm': '%d minutos',
+    'h': 'una hora',
+    'hh': '%d horas',
+    'd': 'un día',
+    'dd': '%d días',
+    'M': 'un mes',
+    'MM': '%d meses',
+    'y': 'un año',
+    'yy': '%d años'
+  }
+};

lib/i18n/fr.dart

@@ -0,0 +1,75 @@
+final Map<String, dynamic> locale = {
+  'Name': 'fr',
+  'Months': {
+    1: 'janvier',
+    2: 'février',
+    3: 'mars',
+    4: 'avril',
+    5: 'mai',
+    6: 'juin',
+    7: 'juillet',
+    8: 'août',
+    9: 'septembre',
+    10: 'octobre',
+    11: 'novembre',
+    12: 'décembre'
+  },
+  'MonthsShort': {
+    1: 'janv.',
+    2: 'févr.',
+    3: 'mars',
+    4: 'avr.',
+    5: 'mai',
+    6: 'juin',
+    7: 'juil.',
+    8: 'août',
+    9: 'sept.',
+    10: 'oct.',
+    11: 'nov.',
+    12: 'déc.'
+  },
+  'Weekdays': {
+    1: 'lundi',
+    2: 'mardi',
+    3: 'mercredi',
+    4: 'jeudi',
+    5: 'vendredi',
+    6: 'samedi',
+    7: 'dimanche'
+  },
+  'WeekdaysShort': {
+    1: 'lun.',
+    2: 'mar.',
+    3: 'mer.',
+    4: 'jeu.',
+    5: 'ven.',
+    6: 'sam.',
+    7: 'dim.'
+  },
+  'WeekdaysMin': {
+    1: 'lu',
+    2: 'ma',
+    3: 'me',
+    4: 'je',
+    5: 've',
+    6: 'sa',
+    7: 'di'
+  },
+  'AM': 'AM',
+  'PM': 'PM',
+  'RelativeTime': {
+    'future': 'dans %s',
+    'past': 'il y a %s',
+    's': 'quelques secondes',
+    'm': 'une minute',
+    'mm': '%d minutes',
+    'h': 'une heure',
+    'hh': '%d heures',
+    'd': 'un jour',
+    'dd': '%d jours',
+    'M': 'un mois',
+    'MM': '%d mois',
+    'y': 'un an',
+    'yy': '%d ans'
+  }
+};

lib/i18n/ja.dart

@@ -0,0 +1,59 @@
+final Map<String, dynamic> locale = {
+  'Name': 'ja',
+  'Months': {
+    1: '1月',
+    2: '2月',
+    3: '3月',
+    4: '4月',
+    5: '5月',
+    6: '6月',
+    7: '7月',
+    8: '8月',
+    9: '9月',
+    10: '10月',
+    11: '11月',
+    12: '12月'
+  },
+  'MonthsShort': {
+    1: '1月',
+    2: '2月',
+    3: '3月',
+    4: '4月',
+    5: '5月',
+    6: '6月',
+    7: '7月',
+    8: '8月',
+    9: '9月',
+    10: '10月',
+    11: '11月',
+    12: '12月'
+  },
+  'Weekdays': {
+    1: '月曜日',
+    2: '火曜日',
+    3: '水曜日',
+    4: '木曜日',
+    5: '金曜日',
+    6: '土曜日',
+    7: '日曜日'
+  },
+  'WeekdaysShort': {1: '月', 2: '火', 3: '水', 4: '木', 5: '金', 6: '土', 7: '日'},
+  'WeekdaysMin': {1: '月', 2: '火', 3: '水', 4: '木', 5: '金', 6: '土', 7: '日'},
+  'AM': '午前',
+  'PM': '午後',
+  'RelativeTime': {
+    'future': '%s後',
+    'past': '%s前',
+    's': '数秒',
+    'm': '1 分',
+    'mm': '%d 分',
+    'h': '1 時間',
+    'hh': '%d 時間',
+    'd': '1 日',
+    'dd': '%d 日',
+    'M': '1 か月',
+    'MM': '%d か月',
+    'y': '1 年',
+    'yy': '%d 年'
+  }
+};