I have 50LB671v and also have trouble with 3D subtitles. Acording to SmartShare's Help it does support, but TV doesn't play internal or external subtitles:
4-7. I cannot see the subtitles of the video I am playing.
If you want to play a video with subtitles, check the following matters.
① Both the sender and the receiver of a video with subtitles should be LG products.
(LG PC, LG TV, LG Net Hard, etc.)
② The name of the video file should coincide with that of the subtitle file, and they should be located in the same folder.
③ Supported formats for subtitles
• With LG PC SmartShare : .smi, .srt, .sub, .***, .txt, .ssa, .psb
• With LG TV : .smi, .srt, .sub, .***, .txt, .ssa, .psb
• With LG Nethard : .smi
④ Among LG PC SmartShare devices, subtitles work correctly with .avi and .mkv video types.
The subtitle will not be played unless all the above conditions are met