Hello, You can try to install the [https://addons.mozilla.org/firefox/addon/quick-accept-language-switc/ Quick Accept-Language Switcher] add-on that allows you to quickly change the ''[https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Language Accept-Language]'' request HTTP header from the toolbar. There may be more add-ons that allow you to specify particular URLs but I couldn’t find any, if supported by 57+ at all. Do not get mislead by e.g. the [https://addons.mozilla.org/firefox/addon/user-agent-switcher-revived/ User Agent switcher (revived)] add-on that allows specific URLs, but changes the user ''agent'' string only. I tried the first add-on myself and it seems to work for reported user agents (you can test e.g. [http://www.reliply.org/tools/requestheaders.php here]) and one specific website tested with, but results may vary. Remember that cookies can keep a previous locale’s view, so you may need to clear them too in Firefox after changing the locale code. I was hoping you could work around such issue by opening specific websites in a separate [[Containers|container tab]], but the customized header persists for any of them, so unfortunately not. If you can’t find any better add-on for your needs than this one, you could of course open an issue/request for adding a domain-specific setting [https://github.com/callahad/quick-accept-language-switcher here].

Hello Tonnes, Thanks for your reply. I tried to find an Add-On for my needs, but apparently there's none. However, the Quick Accept-Language Switcher does not improve or speed up my specific needs. However, I think I can live by manually changing the URL address bar for now. Thanks again for your efforts!

Changing UserAgent locale doesn't really help, at least on TechNet. You can change the language on the top-right corner of this page.