Adding a foreign language to any choir can be daunting for you and the choir, but it doesn't have to be. People think that they have to understand each word of what they are saying in order to sing it. That's just not so. In fact, while you are singing English, if you are singing correctly, you are making "phoneme" sounds and not words, anyway. You're saying, "What the heck are Phonemes?" Phonemes are the basic units of sound that make up language. Phonemes are different for each language, though related languages will share phonemes to a certain extent.
You, as the director, will want to start by working through all of these steps yourself so that you are comfortable with the piece before you even begin it with your choir. If you don;t speak the language you are trying to sing, it is OK. You can still find a language dictionary (or use one of the on-line translators) to get the gist of what you are singing. That way you can communicate it to the choir as they learn.
When you attack a foreign language, the first thing to know are the vowels. Vowel sounds are crucial to making the phonemes sound right. In all the Romance Languages (so called because of their relationship to Latin, the language of Rome, not because they are "so romantic"), those vowels are identified this way (roughly):
a= "aw"WARNING: this "aw" should sound like "Aw...what a pretty baby!"
e - "eh"
i - "ee"
o - "o" - WARNING: this should not sound like the o in "Ohio", but o as in "for"
u - "oo" - WARNING: not "eww", but a rounded stable "oo"
These are sometimes called the Italian vowels. They work for almost any language in some form, though. I even use them in my German to some extent.
Once you have identified, the vowel sounds that exist in your piece, it is time to add the consonants. This is different for every language, but since I am speaking mainly to church musicians, you will want to lay hands on a copy of "Latin Pronunciation According to the Roman Usage". This is the "official" text for teaching Latin Pronunciation. Pronunciation guides exist for ever language under the sun. Since you are on the internet, just pull a search for the language you need to have. You are bound to find something.
Once you have mastered the consonant sounds, it is time to teach your choir to identify the vowels and consonants and how they sound. First of all, make them think like a toddler. Toddlers don't think in whole words or sentences. They think in sounds. Your goal, right now, is not to make them fluent. It is to make them beautiful. Fluency will come later. Once they have a handle on the sounds, start putting it all together.
Pronounce, in rhythm, the syllables of the text and have them repeat it back to you. Try to take small sections at a time. A whole sentence might be too much. Once they have been through it once, do it again, taking larger sections at a time, but still speaking in rhythm. The next time, have them read it with you. Then do it one more time. Finally, put the music with it.
Don't make the mistake of starting with the music, on "La". Always start with the words. Even if you don't understand them, the words give the music animation. Choral music is the marriage of music and text. The two cannot be separated and still be the prayer that it was meant to be.
Another thing I keep hearing with music in Latin is that it is all SO SLOW! Stop it! Do you think that just because it isn't in English, it can't be jubilant? I had a priest tell me one time that I was rushing things too much. (I probably was.) He explained that the ethos of the Mass is that it is timeless, like the worship of heaven. Isn't that beautiful? But while you don't want to rush; you don't want to drag, either. Latin does not equal slow.
Don't be afraid to try something new (or something old that's new to you) just because it is in a foreign language! You will be amazed at how much better your choir will get from the practice of doing something unfamiliar. Singing in a foreign language encourages them to pay more careful attention to technique and to the conductor and that will help their singing overall! That is why in voice school, they teach us to sing all that funny Italian stuff.
Try it! Do it right! They will grow by leaps and bounds!