I'd hardly call it just 'using the language of the western allies' considering it was always used extensively in terms of inward propaganda. One of Hitler's major promise was reversing Versailles, which included the territorial loss Versailles represented.

You are right that after they were at war with the Allies they didn't use that justification anymore, but that was probably in part because at that point they already occupied all of the territories they could even possibly claim as belonging to Germany. I still am not sure I'd say they had 'next to no' justification. They of course used the "Lebensraum" bit were they argued Germany needed more territory for the 'Arian' race. But they also used standard red scare tactics. The official justification for Barbarossa was that the Soviets were planning to attack the nazis (which, given a couple more years, very well might have happpened).