In “Mr. Bachmann and His Class,” one classroom becomes a portal to the world at large. In director Maria Speth’s sprawling, inspirational second documentary, the filmmaker presents about six months in the academic life of one of her personal friends, 64-year-old Dieter Bachmann, who teaches a primary school class for immigrant children in a mid-sized German town. On display are mild culture clashes, linguistic barriers, inquiries into religious differences, debates about values and ethics...