x% is always spoken as "百分之x "（bai3 fen1 zhi1），while written as x%, the same as in English.
As for your example "I am 80% deaf in my left ear",it should be translate as " 我的左耳聋了百分之八十“（wo3 de4 zuo3 er3 long2 le4 bai3 fen1 zhi1 ba1 shi2).
While I have to say, it is not the typical style of Chinese. We seldom use such "exact" numbers during our daily life. In this case, a Chinese should like to say :"我的左耳差不多全聋了”（wo2 de4 zuo3 er3 chao4 bu4 duo1 quan2 long2 le4) , which mean "My left ear is almost deaf"
subtitles = 字幕 (zi4 mu4) -- Rivenlare is correct.