This filter for the LifterLMS Plugin lets you change the text of the Mark Complete button, but only for a specific course.

edit-mark-complete-btn-text.php

<?php // don't copy this line!!!

/**
 * Displays custom text for the Mark Complete button but only for a
 * specific course
 *
 * Added: 2-17-2017 Tyler Kemme
 */
function custom_mark_complete_text( $button_text, $lesson ) {

  // CHANGE THE TEXT FOR THE MARK COMPLETE BUTTON HERE!!!
  $custom_buttom_text = 'Custom Mark Complete Text Goes Here';

  // ID of the course containing this lesson
  $courseId = $lesson->get_parent_course();

  // Only change the button text if this courseId matches the courseId
  // of the course we're going to change
  //
  // In this example, only the course with the id of 55 will get the custom button text
  // we made above
  if( $courseId == 55 ){
    return $custom_buttom_text;
  }

  // All other course get the default mark complete button Text
  else{
    return 'Mark Complete';
  }

}
add_filter( 'lifterlms_mark_lesson_complete_button_text', 'custom_mark_complete_text', 15, 2);