Slack now supports multiple languages, and your app or integration can too.
Add a locale in user and conversation by using the include_locale=true parameter when requesting a bundle of Web API methods supporting it.
The locale field is a string containing a IETF language code, such as en-US. fr-FR, es-ES, or de-DE, and other future values.
We now support include_locale on these methods. Opt-in to receive a locale field in user profiles or the locale of a conversation.
If you don't include the parameter, no locale will be provided. It's opt-in.
You'll also see locale information changes in related user profile change events in the RTM and Events APIs.
We're not forcing locale fields on you. If you don't opt in you won't see them, except maybe in user profile change events.
Your apps and bots cannot yet declare their locale.
Nothing really. You and your app won't "know" the locale of a user or conversational space. You won't be able to customize your app's experience for them. Your app or bot will just speak in its native tongue.
Start sending the include_locale parameter to relevant methods. Consume the locale value and customize experience, like the language or maybe cultural references to be appropriate for that region, culture, or language.
Stay tuned for best practices in app and bot internationalization.
This all started happening on September 12, 2017.
Let us know in whatever language you like if you have any questions or concerns.