Noble titles used by the Galactic Empire are derived from the German/Prussian system. 

Noble Titles Table 

Notes

Marquis and Marquise are French spellings.

The series uses Kaiser for Emperor; but doesn't appear to use the German words for other ranks such as Marquess, Prince and so forth. 

Also Legend of the Galactic Heroes does not appear to use King, Viscount or Baronet at all. The exception is the German Ritter, when part of a last name, indicates the German equivalent of a baronetcy.

Obviously what we really have is a Prussian, German, Norse Myth mix and that makes having accurate and flowing Imperial 'continuity' difficult.

On the subject of heirs, the prefix Erb does not appear to be used at all in LoGH. Erb in German (in this sense) means "hereditary"[. . . .] The oldest son and heir of a Mediatized Count would be an Erbgraf. The oldest son and heir of a Grand Duke would be an Erbgroherzog. And so on. Another way of spelling the title would be Erb-Prinz or Erb-Graf, etc. The wives of these men have equivalent feminine titles, such as Erbprinzessin, Erbgrdfin, Erbgroherzogin, etc. The French form is prince heredetaire, comte heredetaire, grand-duc heredetaire, etc. (toss in accents as appropriate).

Here is an interesting note on the subject of Kaiser Friedrich:

Kaiser Friedrich was also King Friedrich III of Prussia; Prussia is wholly defunct as a state since the Potsdam Conference following World War II. The present head of the (imperial) House of Hohenzollern is Prince Georg Friedrich (born 1976); he is the great-great-grandson of Wilhelm II through his father and grandfather, both of whom were named Louis Ferdinand, and his great-grandfather, Crown Prince Wilhelm. There are Hohenzollerns in the legitimate male line senior in descent to Georg Friedrich, but they have been denied succession rights due to unequal marriages.

Researcher: Lee Thompson

genobles.htm / October 16, 2000