in Functions.js file:

EDIT:
  defaultText: 'Navigate to...',

IN:
  defaultText: mobile_menu_title.mobile_menu_title,

Then in header.php please add next line below "<?php" around line 252 :
 wp_localize_script( 'functions', 'mobile_menu_title', array( 'mobile_menu_title' => __('Navigate to...','cosmotheme') ) );