Now with Firefox ESR 52.6.0esr-2, I did some other tests, each time with
a fresh profile:

1. It seems that language header is not always sent (as reported by
websites that display headers)

2. Giving a default setting (with any language) in a .js file, while
being applied in about:config, does not help.
Example: pref("intl.accept_languages", "fr,es");

3. But enforcing a setting the same way, while not nice for the user, works.
Example: lockPref("intl.accept_languages", "fr,es");

Thanks for your work,

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