Add i18n

[LucaBernstein]

• As comands are valid for each user, there are multiple ways on how to implement this
  • Bind commands in multiple languages simultaneously
  • Only use .OnText binding, check user language (cached?), lookup command and execute. Might require a custom command handler, we might even be able to reuse the subcommands handler.

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• Check for possible library to use, e.g. https://github.com/nicksnyder/go-i18n
• translation files in an i18n/translations directory
• Translate existing project (extract EN)
• Check that everything works for one langauge
• Add another language (DE)
• Make configurable in /config
  • e.g. /config lang [de|en|...]
• implement PoC regarding handling translated commands for both possibilities suggested above
  • decide for one possibility
• implement decided solution, write tests

[LucaBernstein]

Also translate commands?