How to set a default language for booking guests
This assumes you already know how the booking page and widget language picker works for guests, see multiple languages for your profile first.
Can you set one default language for all guests? There is no setting in your Resos profile that pins the booking page to a single language for everyone. A first-time guest gets their browser language. If they have picked a language before, the booking page remembers that choice on their device and uses it again. Either way the choice is the guest’s, not yours.
If you do want everyone to land in the same language, you can do it through the link you share. Add ?lang= and the language code, for example da for Danish, to the end of your booking link, and it opens in that language whatever the guest’s browser is set to:
https://yourrestaurant.resos.com/booking?lang=da
If your link already has a ? in it, join the two with & instead, like ?experience=EXPERIENCE_ID&lang=da. An unknown language code falls back to English, so check the spelling.
The guest can still switch language afterwards with the picker.
The booking widget on your own website works the same way, except the language is already built into the code you copied from Resos. It is set to the language your dashboard was in at the moment you copied it, which is why a widget can keep opening in one language for every visitor. To change it, set your own profile to the language you want, copy the widget code again, and replace the old code on your website. If you are comfortable editing the code on your site, you can also just change the data-lang value in it, for example data-lang="da".