If a Jewish man does not understand Hebrew, is he required to pray while attending services?
Yes. Somebody who does not understand Hebrew should pray in the language he speaks and understands. Prayer in a foreign language is a valid form of prayer. Over time, it is preferable to learn the Hebrew tongue, and to pray in the original Hebrew, which has a special virtue.
Sources: See Shulchan Aruch, Orach Chaim 101:4, and Mishnah Berurah.