Subtitles are hit and miss on Smart TVs. When folk demand they show and the TV doesn't you use HARDSUB to make it happen.
As to Samsung adding support. Never saw them do that. The company is pretty busy this year and so far, if it's not in the product manual, they don't add it.
I found many similar topics but I still have problem with my UA55KU6500 TV :/
I have many movies on my WD Cloud in *.mkv format... and they work perfect on my Samsung tv but I cannot turn on subtitles... I tried everything... the same name, different formats, names of files... Still I cannot get it
I think is impossible that this TV is not "smart" enough to play subtitles.