Square Enix has announced that it will be releasing a demo for Dragon Quest XI S on the eShop ahead of the games September 27, 2019, launch. Revealed at the Dragon Quest Summer Festival 2019, no date is currently planned but some special bonuses will be unlockable for people that download the demo. Users that keep their save data will be given a "Skill Seed" bonus when playing the full version. You'll also be able to unlock a premium hero card in the upcoming Dragon Quest Rivals card game on Switch.
As well as that, some more information about the main game was revealed. In a special bonus, the world of Dragon Quest X will be available for players to visit in XI S. Character Erik will be able to wear this "North Leather Style" outfit from the soon-to-be-released one-shot manga by Hiro Mashima (which releases on August 21 in Japan). This outfit will be obtainable by redeeming a download code. Finally, an enhanced "Draconian Quest" will be present in XI S and will give all party members the "Super Shypox" affliction.
While all of this news pertains to the Japanese release of Dragon Quest XI, the game has been confirmed for a worldwide release in September. I think it's safe to assume that the demo will be making its way West, even if that specific costume does not.