The button no longer works after upgrading to Gotham.
On Windows go to C:\Users\<Name>\AppData\Roaming\XBMC\addons\skin.re-touched and in the folders 16x9 and 4x3Hirez rename DialogSubtitles.xml.xml to DialogSubtitles.xml (remove the extra .xml at the end).
Source: http://forum.kodi.tv/showthread.php?tid=196110
