Fix deprecations + Update WP compatibility

• Fix PHP 8.2 deprecation notice on mb_convert_encoding.

Lower the permissions & hide on front end

• Lower the permissions check - anybody with "read" permissions (e.g., subscriber) can change their locale with the dropdown when logged into the admin.
• Hide the language switcher on the front end - the top admin bar is affected by site language, not the user's choice (opened trac ticket for that)

Change textdomain

• Replace textdomain kbnt-salc with simple-admin-language-change to make it work with GlotPress (more info)

Security patch

• Fix security issues
  • Check for the empty nonce.
  • Escape translations.

Security fix

• Fix security issues (thanks @erwanlr from WPScan for reporting!)
  • Check for the empty nonce.
  • Check for user permission within ajax request.
  • Retrieve current user ID within the request.
• Make error messages translatable.

Use default WP function & pull out the switcher

Drop the old functionality replaced by an integrated solution since WordPress 4.7
Add a simple select box into the admin bar instead.

1.0.0

veversion 1.0.0