Change Default Audio and Subtitles
You can select the audio and subtitles that will play by default on your account from the website: https://uk.rakuten.tv/. First of all you have to Sign In with your email and password. After you Sign in, you have to go the upper right corner of the web page, to unfold the menu of your account, and click on the option "Settings".
Inside the Settings menu, access the tab Audio and Subtitles, where you can choose the Audio language and the subtitles in which your Movies and TV Shows will play by default.
Change Audio and Subtitles while you are playing a movie
You can also change Audio and Subtitles during the play. To do this you have to click on the bubble button on the bar that you will find on the lower part of the screen. All available audio and subtitles options will show up. Remember to click on Continue after choosing.
Check available Audio and Subtitles options
To check if a movie is available on the audio and subtitles option you are looking for, go to the information page of the movie and scroll down a bit to check the audio and subtitles available: