To understand the necessity of "Game of Thrones non-English parts subtitles," one must first appreciate the languages themselves. Unlike gibberish or "movie-speak," the languages in Game of Thrones were fully fleshed out by the Language Creation Society, most notably David J. Peterson.
SDH includes everything: [door creaks] , [speaks Dothraki] , and English dialogue. Forcing SDH might show two lines of text on screen (one from the video's burned-in subs and one from the SDH file).