Fully implement i18n

[tazz4843]

Translations are implemented in nearly everything as it stands, except for the following:

• Command names and descriptions (this includes arguments) (waiting on Slash command localization serenity-rs/serenity#1921 [merged] and Localization serenity-rs/poise#82)
• The voice event handler (this will be an interesting challenge to set up)

[tazz4843]

The Poise PR is blocked on serenity 0.11.3: quote:

This PR depends on serenity's localization support and will therefore be merged once serenity 0.11.3 releases

[tazz4843]

The poise PR is now merged, so work on this can actually begin.