I've translated it as Imperial Duke (国公) of Protection (镇) Or Imperial Duke (last name) depending how the author refers to the character
You could break it into two titles (sounds a little formal and wordy, would not use regularly): Duke [name], Protector of the State
Since it keeps being mentioned, I plan to keep the chinese term as to avoid being too wordy and sounding weird. But for description, all of what you suggested sounds much, much, better than what I had in mind. Thanks everyone for the responds!
"公" combined with the character's name can be an informal title, but whenever you see "国公", it's the official title of "duke". The "镇" will either be his actual name or it's part of the title so it's probably not something you should translate literally. It's like how a title like "镇王" shouldn't be translated literally.
Here's the correct answer (from A Dictionary of Official Titles in Imperial China): So it should be Duke of Zhen.
No problem! There's an online version of this dictionary here: https://projects.iq.harvard.edu/files/cbdb/files/hucker_official_titles_ocr_searchable_all_pages.pdf It's searchable text. If you search by Chinese characters make sure you use traditional characters to search. Romanization is Wade-Giles. Anyway, I recommend this to every translator. Comes in handy, I reference it all the time. The print book version has more info before the dictionary section about administration, civil and military, worked in each dynasty. The online version linked above does have some errors though (wrong characters used, spelling errors, etc). EDIT: Turns out 鎮國公 has its own entry: CH'ING: Defender Duke, 7th highest of 14 titles of nobility granted to male descendants of Emperors; granted to heirs of Beile Princes (pehtzu). The heir of a Defender Duke became a Bulwark Duke (fu-kuo kung); all other sons became Supporter-generals of the State {feng-kuo chiang-chŭn), and all daughters became Township Mistresses (hsìangchūn). BH: prince of the blood of the 5th degree. Dunno if this is more appropriate for what you're translating, but it should be clear by context.
Just wanted to add, the entire title is 郑国公. The two translation as you want to use it in English would be wrong. Duke Zhen /Zhenguo= a general reader will think Zhen /Zhenguo is his name Although it literally means “defender of the state duke” i would recommend that you just translate it as Duke and add a footnote or something, as it just sounds better in English and the general reader will not care about the real accurate title. Grand Duke or Archduke are alternative titles you can use of you want to differentiate it from normal dukes (wikipedia tells me that archdukes are debatably above grand dukes) Actually it's quite interesting , as depending on which dynasty you're in, the meaning of this title changes a bit. As i have doubts that the novel you are translating is set in Qing dynasty, u probably shouldn't base it off Qing dynasty's meaning Baidu page if you wanted to to read it the history, https://baike.baidu.com/item/镇国公
This isn't all that uncommon either. I've been reading a book that uses the Song dynasty as a template but the writer didn't have the guts to make it actually about the Song dynasty despite including a whole bunch of historical figures like Yue Fei and Wanyan Aguda. To be a bit fair, this particular book does deviate from the historical facts (but not by that much).
Yep, for some reason both titles that I'm currently translating used Qing dynasty as references, but doesn't adapted it fully