After you have created a language version, the viewer has the option of selecting the language in which they would like to watch the video in the player. This is done via the headphone symbol at the bottom right of the player.
You can also make a default setting for viewers via the link sent. Depending on the link variant used, the corresponding audio track is then played. However, this can then be changed by the user. You will find the links on the bottom left side in the "Player" area.