Once you started to play a video on mobile app tab the screen to pause. When on pause you will see a "globe" icon in the left upper corner. Please tab this icon for language options. You will get all possible language versions displayed in your territory. If only one language option is visible it means there are no other language versions available for the particular title.
On web the language option is located right above the screen marked by a little flag symbol. Just hover your mouse over the icon and if the video has different language versions you will be able to click on that symbol and a drop down menu will unfold where you can choose the right language or subtitles.