Actually in older times, between 1670-1750, pirate/pyrat were interchangeable. In some songs or shanties pirate is sometimes written as pyrat. Either way, same pronunciation.
Daniel Defoe, I'm sure you've heard of the name, wrote a book titled, "A General History of the PYRATES"
It was written in 1724. It seems both spellings may be used, because there are copies with the word 'Pirates' and 'Pyrates'. Copies made later than the 18th century will mostly have "Pirates" instead of "Pyrates". It just goes to show how language changes over time, and believe it or not, this is the type of magical world Graal is. We're in that whole 1600 to present time.
Remember that Stephan is French. "Pyrat" may be more of a norm to them when writing in English.