Agreed. I also recently found out that in medieval England,
Katherine was pronounced Katerin, without the 'th' sound (this fits in with the French pronunciation too). Soon after, I visited the grave of
Katherine Parr,
King Henry VIII's sixth and final wife who survived him, and the tomb was engraved "HERE LIETH QUENE KATERYN". Perhaps that's where the Welsh form
Catrin came from, too.
Kathleen, as an English name, is relatively recently imported from
Ireland (19th century?). The 'tl' sound in the Old French Cateline must have changed in an Irish accent to create the "Coshleen" type pronunciations found in
Ireland. And then these pronunciations were then approximated into the anglicized version
Kathleen.
I wonder if somebody knows why so many French names in particular made it into medieval
Ireland? Apart from Cateline/Caitlin, I can think of Jeanne/Siobhan, Jean/Sean... I'm sure there are more but I can't remember. I know that anything with a 'j' or 'zh' sound got turned into a 'sh' sound.
I suppose Caitlin/Katelyn is a new step in the proliferation of
Katherine variants.
Caitlin has now given birth to two English variants,
Kathleen (an approximated pronunciation), and
Katelyn etc (a spelling pronunciation - albeit a very pretty one).