Next question: I'm playing Germany, and decisively won the First World War. I noticed that the pool of generals never changed, with Hindenburg, Ludendorff, and the gang continuing to lead my armies into the 1940s, with no new generals coming up the ranks. I thought this was unfortunate, both for flavor and because it meant that I never got any Panzer Leaders and the Air Commanders remained very few and almost totally unskilled, but it wasn't a big deal. It did look as if countries that weren't a part of the Central Powers were updating their generals, just not my side. I don't know whether that's a function of being Germany and Austria-Hungary, or whether it happens to whichever countries are human controlled.
Then, on 1 January, 1960, all of my military leaders retired and no one replaced them. It's kind of hard to run a war with no generals. Fortunately, I have France and Russia as puppets, and they still have officer corps, so, as long as I include some of their units in every attack, I have someone in charge, but it's a real pain.
Edited to Add: It looks like the German leaders are being pulled from the file U08 in the Mods/TGC 4-19-2018/db/leaders folder, rather than the german file, and only the World War One leaders are in that file. The same thing appears to be true of Austria-Hungary (U09) but not the Allied countries. Maybe the solution is to add the leaders from the german file to U08 and austria to U09?
Hmmn. That's still not working. I've copied the leaders from the austria file to the U09 file, changing the country code from "AUS" to "U09" and the Retirement Date to 1999, but they aren't showing up as available in the game.