Add localization

[Smephite]

Add localization and force Philipp to translate.

How do we choose which language to use?

[Triton171]

Maybe on a per-channel or a per-server basis? Per-channel might be annoying if there are multiple channels where the bot is used by the same people but would allow more granularity for large servers.

Also, it's unclear which language should be used in DMs. It might make sense to use the language of any server that the user is part of (that might be ambiguous though, if the user is in several servers) and allow the user to override it manually.

Having the language setting be strictly on a per-user basis would simplify all of those issues, it might be kinda annoying that each user has to set the language.

[Smephite]

Another problem with having the language selection on a per-user basis would be clashing settings in a channel environment.
Would the main server language be used? The language the invoker uses?

I would suggest defaulting to English as main language. If a user chooses to change their language in DM environment -> save it.
Is that same user talking to the bot in a channel? --> default to server setting

Therefore have 3 layers of settings:

• Server Setting
  | > Channel Setting
• User Setting

On another note: Implementing i18n in Rust

https://blog.logrocket.com/rust-internationalization-localization-and-translation/
https://www.reddit.com/r/rust/comments/9uec57/libraries_for_string_localization/

https://projectfluent.org/ looks nice and has a Rust implementation (recommended in the reddit post above)