This feature is not available in RealPlayer. If you are downloading a video, whatever the original language that is associated with the video, will play in that embedded language. This is how it works in the current design.
How can I change the language in a video?
I'm watching an anime that comes with to languages, English and Japanese. The default language of the video is English (regardless of where I open it, I always starts in English). How can I change the language to Japanese? I other players, there's an option to change the videos audio and I change it there.
Thank you for answering so fast!