Translate the rules into your language, and maybe slightly adjust the rules if needed.
Call a few Wikipedians to join you as local organizers (Local organizers (up to five) can get one additional postcard from another country, i.e. if they also accomplish the requirement, they can receive two postcards from two countries)
Post an invitation message on the village pump, Community bulletin board, mailing list, or even make a Site Notice, around two weeks before it starts.
CentralNotice can be translated in the following pages: Add your language code after the page name. Contact revi if you need assistance with editing MediaWiki namespace.