Wagner Street’s namesake becomes clear once you see the 1905 tract map that introduced it (as just one little block southwest of Centinela). Streets on that tract are named after classical composers, including Mascagni, Verdi, and most famously Beethoven. Thus the Wagner in question would be German composer Richard Wagner (1813-1883), celebrated for operas like the Ring cycle, Tristan und Isolde, and Lohengrin. (If you don’t know the titles, I guarantee that you know Die Walküre‘s “Ride of the Valkyries” and Lohengrin‘s “Bridal Chorus”, a.k.a. “Here Comes the Bride”.) I assume the residents of Wagner Street pronounce it the English way: “WAG-ner”. But if you want to be proper, say “VAHG-ner” and see if it catches on.