Site language custom field

Here's a way to enable the Site language selection in the Register and Profile forms:

First, create a dropdown field with a custom choices callback called ` um_051221_site_locale`. Set the field key to `locale` to match the field name in the WP Admin > Users Profile.

Lastly, add the following code snippet to your theme's function.php file or use the Code Snippets plugin to display the 3 site languages in the dropdown:

Now, It should let you change the site language in the Profile form or select it via Register form.