How about allowing for a special parameter ?getslashinfo=1 or something return the various fields that need to be filled in in the Slash Command configuration.
Add a button "Fetch" to the right of the Request URL and if clicked, MM goes to retrieve Display Name, Description, Request Method, Autocomplete Hint and Autocomplete description fields.
Even if the user gets it back in the wrong language, it'll be easy to translate to the local language. Reponse username and icon should obviously not be allowed to be fetched like this, nor the trigger word.
A second parameter could be sent with getslashinfo=1, the system language setting, i.e. lang=en, lang=sv, etc. to allow the slash dispatcher to return the above data in the specified language, if possible.
(Also posted here: https://mattermost.uservoice.com/forums/306457-general/suggestions/19474957-automated-configuration-data-for-slash-command)