The best way to find an English translation of an opera score in the NYU collection is to search in BobCat under the original title of the opera, and then use the facets on the left to limit by language (English) and format (score). In WorldCat you can similarly limit by language and format. You can also often find translated operas under the English title.
Well-known songs or song cycles can also be searched by title and then limited by language and by either score, or audio (for sound recordings).
Sears, Minnie. Song index; an index to more than 12,000 songs in 177 song collections, 1966.
ML128.S6.S439 1925a Non-circulating
(with thanks to Matthew Wise)