Custom registration form setting user locale

Hi everyone
I have created a custom registration form and one of the hidden fields is locale.
When I add a user to DB I add:
add_user_meta($user_id, "user_locale", $locale, false);

This creates a record in wp_usermeta table.
So far so good. The problem is when I look at the new user in wp admin dashboard the language is not set (its just the first option in the dropdown). If I alter the user (approve, deny,..) that first option is updated in the DB. Do I need to set anything else? What am I missing?
BR, Igor

WP version 4.9.4

The page I need help with: [log in to see the link]