Steve Martin's jab at fussy coffee ordering in L.A. Story has given birth to a new java-speak. You hear it in upscale L.A. restaurants: customers trying to mimic Martin and failing. ''I'll have a half caf doublé de cap. No, make that a single caf cap reg cap.'' Those who contend with such orders are a bit muddled too: ''A half double decaf, with a something back,'' says West Beach Cafe bartender Richard Buscher. ''A half caf decaf caf?'' guesses Christine Skaglund of 72 Market Street. The dialogue on-screen runs: ''I'll have a decaf coffee.'' ''I'll have a decaf espresso.'' ''I'll have a double decaf cappuccino.'' ''Do you have any decaffeinated coffee ice cream?'' Finally, Martin delivers the nut: ''I'll have a double decaf, half caf, with a twist of lemon.''