Good question...
First to say you can't liberate Germany as Austria. Once Germany annex Austria or other way around - they become one country. Your only possible solution is to DoW Austria, and annex both with Italy. Then maybe you gain option to liberate Germany (with Austria as part of it).
After all it's a hell of a good question. I'm trying to mod provinces libereted nations gain after liberation, but I can't find where is this defined. It seems it's hardcoded.
Germany is not the only one that doesn't have liberate option. Estonia, Latvia, Letonia cant be liberated after annex either. And some countries are strange in that question. If Soviet go for Molotov -Ribbentrop and gain half of Poland, then later in game conquer German part of Poland, you gain option to liberate Poland - but liberated Poland gain only German part. Soviet part stay Soviet, you must give them rest of provinces trough Open negotiations if you want Poland to be as on the start of the game. If Yugoslavia take Albania from Italy, and try to liberate Albania - Albania get their two provices back plus (!!!) Pristina that is Yugoslavian national province... So you don't want to liberate Albania with Yugoslavia... Pristina even become natinal provice for Albania after liberation, so it's not possible to gain back Pristina trough Open negotiations...
