Selenium change language browser Chrome / Firefox

I'm trying to test my website with Selenium but I don't manage to change the language of the browser. I tried with Firefox, changing the profile also but it's not working.

That's a pity because much of my content is changing regarding the language.

Here is my Python code:

@classmethoddef setUpClass(cls):    super(SeleniumTestCase, cls).setUpClass()    options = Options()    options.add_argument('--lang=en')    cls.selenium = WebDriver(chrome_options=options)

So normally I change the language but nothing happens...

Just to clarify. I already checked on stackoverflow and if I post this question it's really because I tried most of the solutions I saw.