To return, briefly, to the Welsh - if it's in the OED it should also come provided with its own phonetics.
I made the point some time back that it's part of the job and skill of a songwriter to write songs that are singable. And if you have people in a choir (church or otherwise) who are challenged by (a) reading or (b) pronunciation then there's a good case for choosing the songs or hymns with greater care and possibly providing phonetic assistance to enable everyone to participate. I don't think, though, that there's any case at all for altering a language to suit the needs of those individuals! (And what kind of song includes the word "catchphrase", anyway?!)
I also made the point, as have others, that we don't usually pronounce words exactly as they're written in any case, and that's part of the learning process for anyone acquiring language skills. That's not only true of English in all Anglophone countries but also true of other languages. I took part in a workshop last year on singing some Easter songs in Welsh - part of the process was learning to pronounce the words, as well as learning the melody and the harmonies.   I knew the rules of Welsh pronunciation, but there were times when there were variations to suit the melody. It was hard work, but there's nothing wrong with that.