Singers really only need to speak one language, however the reality is that they need to be able to pronounce at least a half-dozen and should learn to speak as many as possible. Even if it's just your basic greetings and vocabulary.
This is not to say, however, that singers should stress themselves out about learning multiple foreign languages. Too many singers stress themselves out about their language skills being not "good enough," as it's easy to forget how much time and practice it takes to learn a foreign language.
What we must learn is the pronunciation of multiple languages and call it diction. We call it diction as singing is basically exaggerated speech, therefore we need to exagerate the pronunciation so it makes sense to you when you hear it with the music. We take diction courses to learn to do this. Art song, opera, and choral pieces are frequently sung in English, French, German and Italian, which are the four foundational singing languages, (in additon to Latin, which we just kind of pick up as we go along). Then there is a whole host of repertoire in Russian, Czech, Spanish, and Hungarian, as well as Vietnamese, Chinese, Polish, Norwegian, Swedish, Dutch...and the list goes on.
Languages are living things, they change with and over time (we know this as we don't speak like Shakespeare any more). Older music frequently has different pronunciation as well as archaic words. In newer music there might be surrealist poetry with altered words or an expression that doesn't translate well. Or maybe an opera librettist may write one character's dialogue in a dialect.
So it's really like any other career, in that you get started with your base knowledge (in this case the singing diction for English, French, German, and Italian) and then you expand it to deeper levels (dialects, understanding foreign language idioms, languages with even more complex pronuncation, rare languages, etc.).
No matter what language we sing in, too, it is the singer's duty to pronounce it as accurately, as authentically as possible and to strive to portray the meaning of every single word. The pronunciation is only half the work. The other half is Part 2 of this article. Stay tuned for part 2, next Thursday!