How to configure Accept-Language?

I'd like to set the "Accept-Language" header before starting my tests so I can test a bilingual website.

Actions.py:181 has "profile.set_preference('intl.accept_languages', 'en')" -- why explicitly set the language to en? w/o this, I reckon I could set the language via the environment?

Is there a way to reset this later, after FF has already spun up, or would you recommend doing this using the browsermob proxy?

I had no success with browsermob using
    # Have browsermob overwrite the Accept-Language header so we can test the page in DE.
    import requests
    requests.post(url="http://localhost:9090/proxy/headers", data={"Accept-Language": "de"})

Any clues?

Thank you all for your time!