Support setting the preferred locale via Guild.edit

Fix #6267

discord/guild.py

@@ -1084,6 +1084,9 @@ class Guild(Hashable):
             The new channel that is used for the system channel. Could be ``None`` for no system channel.
         system_channel_flags: :class:`SystemChannelFlags`
             The new system channel settings to use with the new system channel.
+        preferred_locale: :class:`str`
+            The new preferred locale for the guild. Used as the primary language in the guild.
+            If set, this must be an ISO 639 code, e.g. ``en-US`` or ``ja`` or ``zh-CN``.
         rules_channel: Optional[:class:`TextChannel`]
             The new channel that is used for rules. This is only available to
             guilds that contain ``PUBLIC`` in :attr:`Guild.features`. Could be ``None`` for no rules

discord/http.py

@@ -659,7 +659,7 @@ class HTTPClient:
                       'system_channel_id', 'default_message_notifications',
                       'description', 'explicit_content_filter', 'banner',
                       'system_channel_flags', 'rules_channel_id',
-                      'public_updates_channel_id')
+                      'public_updates_channel_id', 'preferred_locale',)
 
         payload = {
             k: v for k, v in fields.items() if k in valid_keys