There has not been any WDTV updates since 2014 (and probably won’t be)
The ability to download subtitles ‘directly’ has been a feature in XBMC / Kodi well before 2014
(heck, you can even download the subtitles while the movie is playing … and then the subs appear !)
(i no longer use a WDTV … RPi2 with openELEC and Kodi = Great!)