Can't set Member language on custom registration

Here’s my code:

wixUsers.register(field_values.email, field_values.password, {
 "contactInfo": {
     "firstName": field_values.firstName,
     "lastName": field_values.lastName,
     "emails" : emails,
     "language": "es",
      }
    })

Everything works out fine, except the language which I find to be “en” upon inspection of the base Members/PrivateMembersData

I tried fixing the problem by writing a http function with:

let userInfo = {
 "language": "es",
        };
 return wixUsers.updateUserFields(userId, userInfo)

Also works fine when I try changing things like “firstName”, but language seems to do nothing.

https://support.wix.com/en/article/corvid-wix-members-privatemembersdata-collection-fields#language-language

Does this say anything different than english?

https://www.wix.com/corvid/reference/wix-window.html#browserLocale

Also, you might find that the language options need to be set as like in promptLogin.
https://www.wix.com/corvid/reference/wix-users.html#promptLogin

Finally note that you really need to be testing this on a published website as it might not work properly through the preview.

Also, when using Wix Multilingual there are parts that can’t be changed as shown in below link, so take that into consideration too.
https://support.wix.com/en/article/translatable-content-in-wix-multilingual

https://support.wix.com/en/article/request-translating-the-members-area-in-multilingual-sites