Diction guides are found at the call number MT883. Below are a few classics:
Exploring Art Song Lyrics
Call Number: ONLINE
Publication Date: 2012-05-11
Drawing generously from four centuries of Italian, German and French art song, Exploring Art Song Lyrics embraces the finest of the literature and presents the repertoire with unprecedented clarity and detail. Each of the over 750 selections comprises the original poem, a concise English translation, and an IPA transcription which is uniquely designed to match the musical setting. Enunciation and transcription charts are included for each language on a single, easy to read page. A thorough discussion of the method of transcription is provided in the appendix. With its wide-ranging scope of repertoire, and invaluable tools for interpretation and performance, Exploring Art Song Lyrics is an essential resource for the professional singer, voice teacher, and student.
Translations of arias, songs, and sacred vocal music may be found in several resources. Links to the Leyerle (aka Nico Castel) publications and some of the most popular and reliable websites are below. But one can also find translations in other books (scan the Music Library's holdings between ML47 and ML54.8), the liner notes to sound recordings, and select scores. Western Libraries also provides access to bilingual dictionaries between English and several other languages both in print and online.
IPA resources published by Leyerle (Nico Castel et. al.)
These volumes include translations and set the standard for IPA (International Phonetic Alphabet) for singers. Most of these titles are found on the Library's "singer's table"
LiederNet Archive (formerly the Lied and Art Song Texts Page)
An archive of texts used in classical art songs, choral works, and other types of classical vocal pieces. The collection currently indexes 135,350 vocal settings and includes over 25,000 volunteer translations to English, French, Italian, Dutch, German, Spanish, Portuguese, and other languages.
Besides providing basic information about various arias, the database includes translations and aria texts for those titles in the public domain. Though inactive since 2013, the database contains 389 aria translations.
Not idiomatic and often not accurate, but worth a try if you're desperate.
If you use a translation from any resource - either for a paper or a performance - remember to cite it!