Text Domain not loaded

Your plugins textdomain ist not been loaded and it returns false! So translation of the plugin does not work.

load_plugin_textdomain( 'wp-sitemap-page', false, dirname( plugin_basename( FILE ) ) . '/languages/' );

Anyway, on the wordpress support site it’s been mentioned that this command should not called on “plugins_loaded”, should on “init” instead:
https://developer.wordpress.org/reference/functions/load_plugin_textdomain/

// *dont* add_action( 'plugins_loaded', 'wsp_load_textdomain' );
// use 'init'
add_action( 'init', 'wsp_load_textdomain' );