I have a funny feeling that some spells were changed during development and something that was initially translated for the wizard, eventually became the priest's spell. And my guess is nobody thought of updating the translations
As for Durance/Niezłomny... Durance is, among other meanings, an archaic form for "endurance", so Niezłomny is a pretty good choice. Not sure if that was the meaning the creators intended but judging from his monologues (he's a talkative fella, isn't he?) it seems quite suitable, given how he's been enduring the trials and all that.