I use Firefox on mac OS 11.5.2 and the system language is English so is the main display language in Firefox (so I see File - Edit-View etc. on top in English) However whenever I try to print something or save a page the window that opens up is in Turkish. I tried to change this from preferences -> setting-> general -> language (which shows English US as default with Turkish as the second on the list) but the "remove" button is not active. The info on Firefox Troubleshooting Information page is like this: (not sure if it is because of one of these) Internationalization & Localization Application Settings Requested Locales ["en-US"] Available Locales ["en-US","tr"] App Locales ["en-US","tr"] Regional Preferences ["en-TR","tr-TR","el-TR"] Default Locale "tr" Operating System System Locales ["en-TR","tr-TR","el-TR"] Regional Preferences ["en-TR","tr-TR","el-TR"] I also tried from Mac System Preferences -> Language&Region -> Apps I tried to add Firefox and select the language but language selection is not active (in gray "System Default-Turkish" is written and the message under that says Firefox doesn't support additional languages. For other apps like Word etc. system default language shows as English no problem. Thank you