I have uploaded the video on YouTube, added the subtitles, but the users don't realize, that they can turn them on. I know there is particular URL parameter for embedded player, but it doesn't work for youtube.com itself. I have tried out http://www.youtube.com/watch?v=wWMMgHobF6g&hl=de&cc_lang_pref=de&cc_load_policy=1and it turns the page in German, but does not turn the subtitles on.How can I force YouTube to use subtitles on youtube.com itself? 
Force subtitles in video on YouTube, if it's not embedded
youtube