I’ve worked with WordPress websites where the frontend content has not been in English but I have yet to run into a situation where the backend is not English. Today I’m working on a dev project where the client needed to duplicate their live site to a temporary staging site for work to be done before migrating it back to the live site. I realized that the WordPress files were in Italian.
Installing WordPress in another language is pretty easy actually. Head over to the WordPress codex to read about installing WordPress in your language. In this case I needed an Italian installation and WordPress.org maintains a website specifically for WordPress in Italian where you can download the install files. Follow the directions for installation and you are good to go.