Ah, excellent question!
Hmm... looking at Wikipedia, Ralts is apparently based on "waltz", which is a type of dance. Kirlia seems to be named after Semyon Davidovich Kirlian, a Russian scientist who researched human psychic ability and similar phenomena. Gardevoir seems to be a bit more complicated, so I'll just quote its Wikipedia entry:
Cool topic idea. I like it!The name "Gardevoir" is thought to originate from the words "garde" (in French to care and look over) and "voir" (Also in French, "to watch"). It may also be a portmanteau from the word "guard" and "devoir", which is French for duty or "to have", so it could mean "duty to guard" or "it has to guard". That may refer to the fact that Gardevoir will protect its trainer no matter what, even if it has to risk its own life.