Allow all servers to change their preferred locale.

[baptiste0928]

Having the guild and user locale sent with interactions is a big step forward for improving bots i18n, but guild_locale (and preferred_locale in the guild object) cannot be used directly by bots as is. According to the preferred_locale documentation:

the preferred locale of a Community guild; used in server discovery and notices from Discord; defaults to "en-US"

There are two major problems for bots to rely on that field for internationalizing guild-wide messages: servers owners are unable to change the language unless they enable the community feature, and the default language is the same for all servers. This forces bot owners to provide an alternative system to change the language, and server owners to configure the language manually in most cases if they do not have an English-speaking server.

Proposed changes

• Allow all guilds to set their preferred language. This would allow bots to rely only on this setting and not implement their own language configuration system. The language would also be easily synchronized between all bots without manual configuration of each.
• Defaults to the guild owner locale. In most case, the guild preferred language is the same as the owner language. This will allow having a correct default value in most cases, and thus avoid unnecessary manual configurations for users.

It might also be nice to allow changing the language per bot in the future.

[erkinalp]

Defaults to the guild owner locale.

To prevent privacy leakage, you should instead use guild creator's locale at the time of guild creation, and not affected by further locale changes of the guild owner or further ownership transfers of the guild owner.