Currently, outgoing Slash commands send in the request body information about the user like userid and username.

However, to properly internationalize a slash command integration (such as https://github.com/jedfonner/MattermostOnFire/issues/2), the receiving system needs to know the user's specified language (from Account Settings > Display > Language).

Please add a "user_language" field to the JSON object passed in the body of the message sent by Mattermost outgoing slash commands. This field should be populated by the user's language (e.g., en-US, es-MX, fr-CA, etc).