What is up with that?
I mean, they're either in some foreign language like Swhalii, Latin, Black Speech (Mordor language), or whatnot or perhaps they aren't really words at all.
As an amateur composer, I understand what you mean.
Here's basically the breakdown of how it works in the music industry:
Choir parts are written for a language that's most "thematic" for the movie.
In Lord of the Rings, the choirs sing in Elvish most of the time.
In most movies with a religious tone, choirs will be singing in Latin.
In many movie trailers and "epic music", choirs often sing jibberish or Latin.
Notice that English/French/German/etc. are very rarely used by choirs in cinematic music. This is because of a few reasons:
1) Historically, choirs rarely sang in English. As a society, we're actually more accustomed to hearing a choir perform in Latin.
2) In their music education, professional choir singers MUST learn to sing in Latin. This reduces the learning curve for pronunciation in the context of choral music. Time constraints are wicked-tight in the music industry. Between this and reason #1, that's why Latin is so common in choral music.
3) The composer doesn't want to distract the listener. If you are paying any attention to what the choir is singing, that's attention you're not giving the actors. If there is even a single spoken word over the background music, that would be a major problem. You NEVER want the music to be competing for attention with the actors, therefore any music designed to be setting the mood will be in another language.
4) The lyrics would be very minimal, to say the least. Most choral music in movies (Lord of the Rings, etc.) has very few different words. If you were hearing that in English, it would not be very interesting, in all honesty. It would never have the depth of a full song's worth of English words. There's really not much benefit to having it be understandable words because so few words are sung.
5) Most choirs are relatively difficult to understand, even if you know the language. Try listening to a song with an English choir that has a heavy/thick instrumental score on top where everything is playing loud. You'll have a hard time making out the words at all. This goes along with #3: IF people are trying to hear what the choir is singing, you're distracting the listener.
An exception to that rule is when a movie is using a modern-sounding "song" (such as one from a pop star, etc.) In this case, the music is there to set the tone of "familiar", "everyday life" and "relatively normal". This is done on-purpose.
