For singers, reviewing the languages we know, and learning others, is a lifelong activity. Ideally, preparing to sing a song in another language involves: writing the words out separately from the music, translating each word and figuring out the overall meaning of each phrase, figuring out how to pronounce each word correctly and notating the pronunciation with the help of the International Phonetic Alphabet (IPA), speaking the words out loud separate from the music, and memorizing the words separate from the music. (Try to do your own translations, so you can really understand the text in your own words.)
But wait, there’s more! Sometimes the sung language differs slightly from the spoken, and a diction coach and a voice teacher can provide extra help with this, as can books on diction for singing. Plus, some lyrics are in a poetic form; analyzing the form (and knowing where the line and stanza breaks are) provides another key to understanding the song. Bonus points if you can befriend a linguist.
Find a dictionary for the specific language that includes IPA/pronunciations.
Get a diction book for singers; here are some I’ve found useful:
John Moriarty. Diction. Boston, MA: E. C. Schirmer Music Company, 1975.
McGee, Timothy, Ed., with A. G. Rigg and David N. Klausner. Singing Early Music. Bloomington, IN: Indiana University Press, 1996. [Covers earlier forms of European languages, not modern.]
IPA Source (many classical music texts, with translations and IPA): https://www.ipasource.com
IPA for English words (both British and American): https://tophonetics.com
For classical songs, some texts and translations: http://www.lieder.net/lieder/
Visit any of these sites for info on poetry and poetic forms (and about Poem in Your Pocket Day!).