The Germany of the game is actually Prussia, which was seperate from the Holy Roman Empire. Frederick the Great anyone. If people want to keep mixing up Germanic peoples and Germany, they might as well put their hand in lava.
I wouldn't be too bothered if the rest followed by the way.
The Germany of the game is Germany. Prussian leaders have been selected by firaxis, but the name and city-list is Germany as a whole.
Prussia wasn't actually separate from the HRE in practice. The elector of Brandenburg wanted the title of king, but couldn't be "King of Brandenburg" because Brandenburg (most of his best territory, where Berlin is) was in Germany; but some of his outlying territory, Prussia (an old semi-dependency of Poland), was outside of the Kingdom of Germany, and so the Emperor allowed him this title. So, yeah, in title, Prussia wasn't in the Empire, but Frederick was a vassal of the King of Germany/Holy Roman Emperor for most of his lands.
Not it anyway... The USSR and Russia is like Britain , the UK and England. There is a huge amount of difference here. The Holy Roman Empire never became Germany, Prussia came about from the East and then proceeded to become modern Germany. The Holy Roman Empire dissolved and in 1870 Germany as it is now was united by the Franco-Prussian war. On the other hand the USSR came about directly from Russia. Now do not say that it became the USSR after the communist take over. The name Union of Soviet Socialist Republics came about as a compromise much like Austria-Hungary so they could keep control of places like the Ukraine and Belorus. They changed them from territories of Russia into Soviet Socialist Republics. It was a direct change unlike that of Germany and the Holy Roman Empire. That and the Holy Roman Empire was Germanic not German... [said once more for your enjoyment] ...Germanic is not just Germany, remember that. Stop making stupid comparisons people... really!
Modern Germany essentially derives from the HRE in a line of continuous development. The HRE, like it or not, was commonly called "Germany" or "the Empire". After the reunification of most of the Empire in 1871, much of the old HRE terminology was revived (e.g. "Kaiser", "Reich", etc), and it was cast very much in the light of a resurrection of the 1st Reich, not as (like English-speaking schoolchildren are taught) a new creation; you will notice that the old line of German kings, now using the title "Emperor of Austria", were presidents of the German Confederation and were always thought of as the leading candidates to resurrect the old state. You remember what the "reunited Germany" called itself? German Empire (Deutsches [Kaiser]reich)! With that the HRE is just smacking you in the face!
Because of Napoleon, the last King of Germany Francis II abdicated in 1806 and dissolved the Reich, but it was kind of revived with the German Confederation (which, like the HRE of the German Nation, included Bohemia - though in pure technical terms, Bohemia was never really part of the Empire
as Germany, few people drew that distinction by then) after the defeat of Napoleon. Brandenburg/Prussia was very successful during the Napoleonic Wars and acquired more territory in the north of Germany-proper, defeated Austria, then France, and reunited most of Germany outside of the Austrian Empire (i.e. mainly Austria and Bohemia) ... which was, unlike the HRE, a proper multi-national empire.
As to "Germanic", no-one knew of any such word back then. It's a neologism, and was never used by anyone until the 18th century growth in linguistic origin mythology/history. You were either German, or you weren't. So, I'd advise you not to overplay that one. It was the Holy Roman Empire of the German Nation after all, not the historically impossible "Holy Roman Empire of the Germanic Nation".
Also, some people are trying to make out that the HRE was some kind of multi-national state; well, it was and it wasn't. The King of Germany first obtained the title "Emperor" while only King of Germany. He became later King of Italy/Pavia and King of Burgundy/Arles; Bohemia was acquired by the Hapsburgs and came into the Empire, but the Kingdom remained the Kingdom of Germany with an imperial gloss. The language was always German, and indeed the formal style of the Empire became "Holy Roman Empire of the German Nation". That is about as multi-national as the Kingdom of the King of England (which came to include Cornwall, Wales and Ireland). As the HRE meant Germanization in places like Bohemia, just as anglicization proceeded in Wales, Cornwall and Ireland, overplaying some "multinational" elements would be critically misleading.