env.rb

Capybara.default_driver = :selenium

it_profile = Selenium::WebDriver::Firefox::Profile.new
it_profile['intl.accept_languages'] = 'it' # Italian Locale

Capybara.register_driver :selenium do |app|
  Capybara::Selenium::Driver.new(app, :browser => :firefox,
                                      :profile => it_profile)
end

Capybara.current_session.driver.browser.capabilities.firefox_profile = it_profile