Instructions
Launch the Netflix app from the Xbox dashboard.
Log in to your Netflix account if you haven't used the app before.
Select any program on Netflix by browsing or using your instant queue.
Press any button on the Xbox 360 controller after the show starts playing to display a menu bar.
Select "Audio and Subtitles" using the directional pad, and then press "A."
Pick the language of subtitles you want from the displayed list, and then press "A." Note that available languages vary by program.
Press "B" to hide the menu bar.