Is this a joke? Not sure. If it's a serious question - the diet was "the diet" - various ways of saying it in Latin (none of which - I'm pretty sure - ended in 'G'). Once the crown moved to Bavaria, it would have been the Reichstag.
A Landtag is the State Parliament of each German "Land" or State; the Federal (i.e. National) Parliament is the Bundestag. My guess is that "Landtag" was also the term used for all the mini-states when there were hundreds of them.