title: Cannot change language of selenium-webdriver for firefox
body: <p>I am running e2e tests based on selenium-webdriver (firefox). Some of my test cases are based on comparing result messages. The problem is when I run tests on my local machine (and my local firefox browser) the language is automatically changed from EN, which ruin some of my test cases (which compare same messages but in different languages).
  Firstly I tried changing it in my browser. I reorder my preferred languages list, so that EN is on the first position. On every test run it all goes back to the original order (with EN on the second place). I also tried removing the language that is being chosen automatically. But after first tests run, the list of languages gets back to it previous form (with EN being second).</p>
  
  <p>Next, I tried setting language for my webdriver. My final code:</p>
  
  <pre><code>let profile = new firefox.Profile().setPreference("intl.accept_languages", "en-US");
  let options = new firefox.Options().setProfile(profile);
  
  let driverBuilder = new selenium.Builder().forBrowser(webDriverConfig.browserName).setFirefoxOptions(options).build();
  </code></pre>
  
  <p>But this take no effect at all. During tests I still get wrong locale and after running tests in my firefox settings I am still getting wrong language at first place (in language preference list).
  How can I set english to be my accepted/preferred language during my tests?</p>