With bug 1595697 we will get the basic implementation of Emulation.setUserAgentOverride, which only handles the user agent.

This bug covers the addition of the acceptLanguage option. Similar to the userAgent the preferred language can be set via a preference. In this case it will be intl.accept_languages. It's a string preference and accepts a list of languages separated by comma. Given that the input is a string it should directly be set.

The files to make the changes can be found at:
https://searchfox.org/mozilla-central/source/remote/domains/parent/Emulation.jsm
https://searchfox.org/mozilla-central/source/remote/test/browser/emulation/browser_setUserAgentOverride.js

To get started please read:
https://firefox-source-docs.mozilla.org/remote/index.html

As it turned out there are two implementations. The one from the Network domain simply redirects to the Emulation domain. Updating the bug and dependencies accordingly.