There is a firmware issue with track selection not automatically happening with MP4 files. Other container types should work. use content. SubtitleConfig to specify track explicitly. Sadly track selection based on preferred language will not work this way.
I just got a new Roku test device. It's a very low powered Roku device (Roku Express). It has the latest 8.0.0 firmware (Build 4128-28). When I click the options button "*", nothing happens. The closed captioning side bar does not appear like it does on my Roku 4 test device. The Roku 4 device has the latest firmware version for that model (Build 4128-28).
It appears for now the old and new ways of handling of closed captioning should be in the same codebase. If the device supports thew new way, then it will simply ignore the old code. If the device doesn't handle the new way, then it will ignore the new way. (Of course, I don't have a device with old firm ware so I am assuming the app will not error out when it encounters the SubtitleTracks property.)