i want to make my own characters
It sounds like you want to make your own fonts. This is not trivial, you'll want to read into different types of fonts (vector vs raster/bitmap, TrueType and OpenType) and then find a program for making fonts. There are free ones out there but I couldn't recommend one, it's been years since I last played around with making fonts,
If you do actually want to make an actual language pack (as in translate the OS to another language), you're out of luck, there's no way to do that on Windows (unless you work for Microsoft).
