Karou

lmao I misinterpreted the title of this thread as you wanting to rent someone's passport XD
 
does your U.S. visa count? 

Haha, it was pretty much the highlight of my day too! I was waiting at the traffic light and this lady was talking to her dog, calling it by its name, and I had to say something because 1. dog and 2. Mustache is a great name (although I think General Custer may have it beat! Outstanding). Petted him for an entire three seconds and yup, pretty much made my day.  

This reminds me of a manager I had back in the day. It was when tiny phones were the big thing (flip phone era). Even with a ridiculously small phone, it still popped out of her pocket and into the toilet... and she flushed it. She didn't realize it had fallen in until it was being sucked down the hole.

Seriously, thats one of the main reasons why I carry a purse. I'm paranoid that its going to pop out and break. I bet Apple is going to make this a feature somehow

Am I a bit late? But: your (written) English is [redacted] phenomenal. You mentioned repeating the process by which you learned English with German. All I can say is that if your language learning process works even almost as well this time, you will be absolutely stone-cold fine.
I am a bit surprised by the Latin/German parallel, but after you made the point about your difficulties with grammar, I am inclined to think that they will both be about the same level of difficulty for you, given that they both have case systems and their gender systems are a bit more complicated than the one in French. Given those similarities, however, I would simultaneously whichever one you learn first will help you learn the other one more easily. 
I do second or third the idea that you should look at what kinds of sources you'd most like to read before you make the commitment to one or the other complete, though. Perhaps looking at the sources in the notes of your several favorite books or articles would be a good idea?

It might be helpful for you to identify what kinds of sources you are interested in studying and to decide on the languages you need from there. Are the 16- and 17th-century texts on prostitution, witchcraft, etc. you're interested in written in Latin or the vernacular? The language requirements exist, at least essentially, to demonstrate your ability to execute the particular kind of research you want to do.
For the intellectual history of early modern Europe, Latin is typically crucial. I imagine beginning there might be best given the scope of your interest. But perhaps your focus on social history leads you more often to German and Italian language texts, so maybe those languages would be more useful to you. I agree with xypathos that it is a good idea to reach out to a professor with this question about how best to prepare yourself for your research interests.

I'd echo @TMP's reflections as well! What languages are required are entirely dependent on the area and time of what you will be studying. If you're doing something specifically on Germany, then you'll need more than a reading knowledge of German. Your French will be a huge benefit (it would almost certainly be required for a program in the US though since it's your mother tongue it's not something you need to worry about) and whatever German you can pick up beyond a reading ability would be highly beneficial if not required.
A lot of programs include language requirements for specific fields on their website. Should it not be there though, reach out to a professor at a potential school and inquire about general expectations of incoming students (languages you're expected to have down before entering) vs. languages they're okay with you having some skill in by year 1-2.

Don't overkill yourself.  Do whatever you can to pass that reading exam.  You'll improve your German as you conduct research.
However, I should warn you that there are high expectations for mastery of German if you intend to research in Germany.  A lot of grants and fellowships funded by the Federal Government (Fulbright, DAAD (German Academic Exchange Service) and humanities-based programs in German universities do expect a certain level of proficiency and language evaluation forms.  One definite exception, I think, is the DAAD's Intensive Language Grant as it sends you to Germany for 2 months to study German language.   Having said all that, if you think you're going to need to spend time in Germany for research, I'd eventually start working on your oral skills, not just reading.
French is excellent to have for diplomatic relations pre-WWII and culture.  However, German was the business language for the sciences, medicine, and I think military.  However, if you're thinking on transnational terms, then yes, you need German if you're looking to work in most of Central Europe to eastern part of Russia.  But you won't really need it outside of Europe unless you're working on ethnic Germans.
(haha sorry i got carried away.. I just love German history!)

How departments certify language ability varies across the states and schools, there's no uniform agreement. That is, generally if there's 2-4 semesters (1-2 years) of a language on your transcript with good grades, then you'll be good to go. Some departments do language tests no matter what though. Some programs will accept participation in an intensive summer language program. Yet again, some departments specifically offer courses that allow you to develop reading competency in a language (with the use of a dictionary) - generally French, German, Spanish, Latin, Greek, and a spattering of Hebrew programs.
If you teach yourself just be prepared to take a language exam - many of us here have done this and it's perfectly acceptable (generally).
I studied Latin in high school and college and found it helped tremendously when studying other Romance languages (I have intermediate French and Spanish, and intermediate reading Romanian (it touches on my research area!)), but found it "relatively useless" when it came to German (admittedly my area requires Low German so a bit different). That said, once you have a system down to learn a language, it helps across the board.