There's a Latina singer named Thalia (tah-LEE-a). I don't see why the names can't be related much the way Theresa and Teresa are. As I'm sure you know, not all languages pronounce the 'th' and I can see how eventually, it was dropped. I also see Talia as having multiple origins.
ETA that I was on another name forum several years ago. One of our regulars was from Peru and her sister's name was Thalia (pronounced like the singer's name).
This message was edited by the author on April 11, 2008 at 4:40:25 PM