Vayra asked some questions. Let's go.
1) What class knows the most martial arts? Are they real martial arts like kung fu, or made up ones like krav maga?
Any sort of Monk-ish type class can learn Martial Arts, as well as fighters that feel like it. They'll be generally realistic, unless the class is specifically attuned to Wuxia tropes. Sufficient spiritual attunement (either having a sufficiently high WIS score, taking levels in a class keyed to those concepts, or having earned such spiritual awakening through play) can give access to supernal martial arts. Stuff an Exalted character would pull off. Kill Six Billion Demons, yo.
2) Can I start out having already made a deal with the devil or do I have to do that in game?
Both are possible.
3) Do you want me to write an 8-page backstory? Can I write an 8-page backstory, if I want to? If I write something down in it like I'm the timelost princess of the brass city and the daughter of the sun and I commanded legions in the Hell War but was betrayed by my father's vizier but I don't know that, or that I'm elf conan and cooler than everyone else, will that be true?
I don't need big backstories, but fucking go for it. If you write an outrageous backstory that doesn't entirely ruin what the other players are trying to do, I'll allow it. I very often do not create my settings until I have the PCs.
4) If I eat someone's heart, will I gain their powers? What about their brain?
Generally, no. This is often a psychic ability or magical ritual that can be performed. However, eating the heart or brain of an inherently magical creature will do weird things to anybody.
5) These classes are boring, can I be one from somewhere else? What about from a different system entirely?
6) If I make a sword, which one of us gets to name it?
7) Am I allowed to kill the other player characters? What would I have to do to be allowed to? Do I win if I kill them all? Actually, how do I win in general?
As a general rule I prefer PCs to cooperate and work together. I don't mind PVP as long as a meaningful justification comes up, like "Hey, Brad literally murdered my love interest, I demand reparation." As for "winning", that's accomplished by retiring your character in whichever way satisfies you.
8) What language stands in for 'Common'? Or what are we all talking to each other in? Like the party, mostly, but also everyone else?
Common works. Who cares, honestly?
9) How do I learn how to talk to rocks? No not once a day just, like, normally?
Be nice to them. Original Sin does not exist in my setting, and so all newborn babies can hear and understand all voices of the stones, the wind and rain, the animals and the trees, the stars and the void between them...
We lose this power as we develop our egos and establish that we are ourselves, and not the rest of the universe around us. We stop hearing the voices, when we forget them. But every once in a while, a sufficiently pure heart never forgets. And the silent sounds of the world are always happy to talk to you again if you remember.
The hearts of people that shoo out bugs instead of crushing them, and those who sing to their gardens, and those who gently return borrowed stones instead of skipping them into rivers... these hearts eventually hear words of thanks. And then hear more.
10) Which kinds of wizards get to serve kings and live in towers and shit and which ones are run out of town or stoned to death in the streets? Can I be both? At the same time?
People generally have a pretty low opinion of wizards, sorcerers, warlocks, witches, all that shit, for causing trouble. Some towns might have tolerated witches or magic-users who don't summon Cthulhus in their basements and use their powers to bring good weather or bless babies or whatever. It is generally these cunningfolk who get approached by kings and such to ask for their services, if they find out about them, or you can be like a wizard who totally saved the king's life or healed his daughter or something, and he offers you a boon, and you ask for a sweet gig in his court or a tower or something. There is, technically, nothing keeping you from being both. I'm sure many a wizard has plagued the child-prince of the lands and then shown up with the miracle cure.
Magical Girls are generally always tolerated by everybody. They're easily identifiable when they're doing their magic, can hide their identities easier than everyone else, and their powers tend to be geared towards helping people. Entire nations have forms of government where their ruling queens are Magical Girls who passed a compassion test.