Wow, it's rather bold to want to try to make a pull request without compiling... you are sure you don't want to compile, even without understanding the c code it should be possible and would make things easier for you. Anyway if you want to try, from memory steps are :
1) take a github account, I think you said you already have one to post comments
2) go to the raine page, and click the "fork" button on the top right, it will propose a name for the copy of the repository on your account, you can accept it, you have a copy in your account now.
3) go to the freshly made copy, and clone it to your disk, I don't think you can edit files directly from the interface of github at least, that's the point where it's much easier if you have the tools for that, usually it means installing git, which is installed by default if you install mingw32. After that you open a terminal, type git clone address where address is the address you get when you click on the big green button "clone" from github on your fork page. Visual studio probably also installs its version of git, but I find there are too many options everywhere in visual studio and it tends to be messy.
4) do your changes. Here again, it's easier if you have some dev tools, I use vim usually in linux but there are other tools in windows for that, I think visual c++ has a good interface to git, but I don't like to use it so I can't tell. These tools make things easy to locate something specific in the sources to change it. Without them, well the main menu is in source/sdl/gui.cpp, and all the other dialogs are in source/sdl/dialogs (the file names tell what is inside !). Your changes are only about changing some strings so it's possible to do it without compiling, but it's bold for sure.
5) commit your changes, it's about using the git commit command, it will ask you a description, validate the description, and it's good. Then git push to push your changes to github, they increased security on github recently so they require ssh keys or other strong methods of authentication for that, they have a dedicated page to explain that.
6) if you reached this point, it's done, you can use "pull request", I don't remember if it's on your fork or raine page, it must be on your fork, it looks at the recent commits and proposes some defaults to you, add eventually some description and it's done.
It would be quite a feat if you succeed to do that, so I'll be forced to accept it of course ! There are ways to view files from the interface but I don't think you can edit them from the github interface, but maybe I missed something. Good luck, if you fail I might look into this later... but if you succeed, you should really follow the tutorial on how to compile to be able to compile your version... !