BRZEGI, Poland — Pope Francis challenged hundreds of thousands of young people who gathered in a sprawling Polish meadow to reject being a “couch potato” who retreats into video games and computer screens and instead engage in social activism and politics to create a more just world. Peppering his speech with contemporary lingo, the 79-year-old pope, despite a long day of public appearances, addressed his eager audience with enthusiasm Saturday on a warm summer night. Francis spoke of a paralysis that comes from merely seeking convenience, from confusing happiness with a complacent way of life that could end up depriving people of the ability to determine their own fates. “Dear young people, we didn’t come into this world to ‘vegetate,” to take it easy, to make our lives a comfortable sofa to fall asleep on.