How do I change the language settings?

My system is stuck displaying everything in American, and I’m desperate to switch it over to English since that’s what I actually use... I went into the settings and found English listed among the available languages, so I tried adding it to my preferred list, but there's a catch. Right above where you select your language preferences—under the section labeled "Display language for macOS"—it only gives me the option for American, leaving me completely unable to select anything else. Does anyone know how I can force this change?...