You can find the full version of the current Firefox release (66.0.5) in all languages and all operating systems here:

• https://www.mozilla.org/en-US/firefox/all/
• https://www.mozilla.org/en-US/firefox/66.0.5/releasenotes/

Is this about the language of the Firefox user interface or about the language choices send to websites as part of the HTTP request headers?

The language of the user interface can sometimes get changed when you update and downloading a full version is necessary. When this happens you need to reinstall the Firefox version of the correct language like I posted above.

If you want to be able to switch between languages for the user interface then you need to install additional language packs. Note that you shouldn't confuse language pack and spell check dictionaries.

• https://addons.mozilla.org/firefox/language-tools/

Updating Firefox shouldn't affect the language settings in Options/Preferences about the preferred languages for website.

• Options/Preferences -> General -> Languages

They want to change the language for many computers/profiles at one command.

Can you clarify what language this is about (user interface or different)?

FredMcD understood that I am looking for a way to apply the same fix to many computers at once.

Currently visiting a computer and selecting Options/Language and then Select English (United States) is second choice

I am searching for a method that I can automatically make the change for all users.

Do you have multiple languages installed via additional language packs on all these computers?

You can use GPO or a policies.json file with this policy. RequestedLocales:

• https://support.mozilla.org/en-US/kb/customizing-firefox-using-group-policy
• https://support.mozilla.org/en-US/kb/customizing-firefox-using-policiesjson
• https://github.com/mozilla/policy-templates/blob/master/README.md

{
  "policies": {
    "RequestedLocales": ["de", "en-US"]
  }
}