I'm having a similar issue lately and it's driving me crazy! My intereface language has always been English(and Language&Text->Language) has English on top).I have installed a Greek layout and a Chinese one besides the English one.
Well, one day I suddenly realized it was changing my language to greek on itsown! So frustrating. Passwords, URLs, every input field on every app switches to greek when clicking on them! I haven't installed any new software the past few months and definitely not any tool tha messes with the languages...
In the "Language&Text->Input sources" I can only select the installed input methods, I can't change their order and greek appears higher than english... When does one change the default keyboard layout for the mac? The has to be a place for that!
Anyway, I temporarily disabled the "Allow a different one for each document" option hoping to at least minimize the frustration...