I'm really not sure, maybe one of the other guys can take a look and let you know. From what I see, I don't see any reason why you can't create the admin account, but I'm not 100% sure.
Once you've changed the default language in the users account, you'll want to clear your browser cookies for your website.
After you've changed the default language, all new users will see the default language, others will have to clear their browser cookies for your site as well.