Next week I'll be singing a bit of Fauré at a funeral - and at the rehearsal I fully expect the leader of the rehearsal to say to the sops 'What are you singing about? In Paradisum. "In Paradise". You sound as if you're singing about what you had for breakfast....' (Conductors mostly seem, in my experience, to regard breakfast menus as the nadir of interest.)
But it doesn't mean that. In can mean many things in Latin, but when followed by a noun in the accusative it doesn't mean 'in'. If the words were In Paradiso they would mean 'In Paradise'; but they are In Paradisum ... going on ...deducant Angeli : 'May angels
Interestingly, deducere can also mean 'mislead', but I doubt if Fauré had this in mind - though Barrie Jones, collector of his letters, doubted his piety (on p. 24 of the 1989 edition). His most pious work, the sublime Cantique de Jean Racine (survivor of many a choir's mispronunciation: Verbe égal aux très-haut: 'Verb equal to thirteen waters...' - give me strength! - and de tes dons qu'il retourne comblés : 'of your teeth which he gives back because they're ... impacted?', was written in his teens). And in his later years he may have taken after his friend and teacher Camille Saint-Saens, who - according to one biographer - prescribed for his funeral a short service, if it had to be religious at all, and proscribed the singing of 'Pie Jesus' (sic - Either that final s is the biography's typo, or it was Saint-Saens's attempt to spare Fauré's feelings: 'I don't mean your setting of Pie Jesu.')
But 'sufficient unto the day is the evil thereof'. The imagined solecism will probably not be perpetrated.
(And that's another thing: perpetrated/perpetuated. But I must stop. Duty calls.)
Update: 11 Nov pm - updated TESconnect stats, and tweaked second para.
Update:12 Nov am - Added to third para. and added the fourth
Update: 2018.06.12.14.55 – Correction to deducant translation.