If you only want Spanish then just change the language in your site.
https://support.wix.com/en/article/changing-your-sites-language
https://support.wix.com/en/article/changing-the-language-of-your-account-dashboard-and-editor
Or through how Mikhail as shown you above with the changing language of login page.
Or look into using Wix Multilingual so that you can have english and spanish dependant on what your user wants.
https://support.wix.com/en/wix-multilingual/wix-multilingual-basics
As for when you create a new members page in the Wix Editor it shows it as english, well it probably will as that is the default automation setup by Wix.
Try changing your Editor language and see if it changes with it, otherwise it will always be shown as english when adding a new page through the Editor.
Like you’ve said above with the prompt password, you can do your own lightbox for login and signup and do it yourself through code too.
https://www.wix.com/corvid/reference/wix-users.html#LoginOptions
Finally, note on that API reference page too…
The two letter language code of the language to show the login form in. Defaults to “en” if the property doesn’t exist or the given language is not one of the languages found in the Permissions tab of the Page Settings panel in the Editor.