Recently I've been listening to the music of Noir, an anime I like (although the ending kind of sucked), its music composed by my beloved Yuki Kajiura. I quickly came to like one song in particular, called Salva Nos. Eventually I started to wonder what the lyrics were (noticing a pattern, here?), as the operatic style makes it difficult to distinguish the words by ear. A quick search on Google turned up a listing on Anime Lyrics (I've modified them a bit because I think there were errors in the original transcript, and I've included my rationale for these changes in brackets).
exaudi nos et miserere [singular imperative form of 'misereor' - have pity]
Dona nobis pacem
et salva* nos a hostibus
Salva* nos, Deus
Dominus* exaudi nos
Dona nobis pacem
dona nobis pacem
et ['and'] dona eis requiem
inter ovas* locum
voca me cum benedictis
pie jesu domine, dona eis requiem
dominus* deus, Sanctus, Gloria
Anyone who has seen such a thing before will immediately recognize this as Latin. It looks like a Christian hymn, based on the subject matter; either a real hymn (as in, written back when Latin was still common in the church) or an imitation made to sound like a real one. At first I suspected it was the latter, until I read the lyrics near the end of the song. Much to my surprise, there was a reference to Jesus by name. This is such a surprise because less than 0.5% of Japanese are Christian, and a good majority doesn't even know who Jesus is; this suggested that perhaps it was a real Christian hymn, after all.
So, how to tell the difference? Well, the first solution that came to mind was to check if it was proper Latin grammar. If the grammar was flawless, it was probably written by someone fluent in Latin, such as a native speaker or a priest of the church. If it was badly bastardized, it was probably written by Kajiura herself (for comparison, take a look at the lyrics to Maze, by the same composer).
Now, checking Latin grammar isn't hard... if you know Latin. Unfortunately, my knowledge of Latin consists of a few dozen words along with my knowledge of Spanish (a Latin derivative, which often resembles Latin). That means it's time to break out my Latin-English dictionary and Quick Study Latin grammar sheet.
Okay, going through this word by word (specifically the ones I marked with asterisks, indicating that I think they're incorrect)...
Dominus: This is the nominative (sentence subject) form of 'lord'. The correct form for direct address would be 'domine'. I don't know whether Deus is kept the same for the direct address form or not.
Salva: This is the Spanish/Portuguese form. 'Salvar' means 'save', and the conjugation is imperative. However, in Latin 'salveo' is more of a passive voice - 'be well' or 'be safe' (it also is an intransitive verb). The correct word in Latin is 'servo', conjugated as 'serva.'
Ovas: A form of 'ovis' - 'sheep' - referring to God's sheep (the church). The correct conjugation would be either 'ovibus' or 'oves' (I don't know whether the accusative or ablative form would be correct, here).
Okay, so, that makes it seem likely that this is not a real hymn; that leaves the question of where the lyrics came from. As mentioned previously, most Japanese don't know who Jesus is, so it would be improbable that this was written by somebody who is Japanese. Well, let's look for other clues...