The Forced flag means to help play this subtitle containing english even though the primary audio in the file is English. When your settings are correct. Plex is just playing file. It can’t pick out parts for you ( dvds and DVD players have special encoding to do that ). You need to have subtitle files that only have dothraki ( foreign ) parts.
such as these
https://subscene.com/subtitles/game-of-thrones-first-season/english/1147545
I like subscene.com because it is nicer to search for things since you can pre-filter the search to only look for those
Same for movies like Dawn of Planet of Apes where you need subs which are only parts apes are doing sign language
