It seems like changing the HTTP_ACCEPT_LANGUAGE is not possible for a headless chrome.

• On Ubuntu the headless Chrome derives the Accept-Language from the operation system
• Adding the option options.add_argument('--lang=de') to the Capybara::Selenium::Driver has no effect
• Adding the preference options.add_preference('intl.accept_languages', 'de') to the Capybara::Selenium::Driver has only effects if the --headless option is skipped (see bug ticket #775911 Show archive.org snapshot )
• Changing the request header in Capybara only works without Selenium

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You still can add logic to your application code that helps to modify the Accept-Language in tests. Here is an example for a Rails application with Cucumber integration tests:

class ApplicationController

  OVERRIDE_HTTP_ACCEPT_LANGUAGE = nil

  def accept_language
    OVERRIDE_HTTP_ACCEPT_LANGUAGE || request.env['HTTP_ACCEPT_LANGUAGE'].to_s
  end

end

# Override the Accept-Language header for all Cucumber scenarios

Before do
  stub_const('ApplicationController::OVERRIDE_HTTP_ACCEPT_LANGUAGE', 'de')
end