It doesn't work like this. You need to create a new file with the country code. You can use an existing translation for inspiration (like in French or whatever) and then try it.

There's no file to modify to change the language. Go in your user zone (left menu, the "user" icon, almost on the bottom, and then you can select your language, that will be immediately set.

You shouldn't assume about the distro used as a base, but you can safely assume about being RPM based