If you're muttering, the spell automatically fails.
"Most spells require the chanting of mystic words. The words themselves aren't the source of the spell's power; rather, the particular combination of sounds, with specific pitch and resonance, sets the threads of magic in motion."
That's very audible and clear, so if the Count can understand the language that the cleric is praying in, he's going to understand, "Oh highest god, help my friend persuade the count to do as we wish." or whatever the prayer is.