百分之八十 = 80 per cent. 字幕 = subtitles.

For percentages, instead of saying "it is X%" like in English, you would be saying the percentage out of 100. (Because 80/100 = 80% yeah?) 80% = 百分之八十 (literally 80 points out of 100) For your example "i am 80% deaf in my left ear", you can simply say 我左耳听力还剩八成 which literally means "i still have 8/10 hearing power in my left ear". You can also say 我左耳失去了二成的听觉 which literally means "My left ear has lost 2/10 hearing power" subtitles = 字幕 (zi4 mu4)

我wo3的de1耳er3朵duo聋long2了le百bai3分fen1之zhi1八ba1十shi2了le 80%=百分之八十（bai3 fen1 zhi1 ba1 shi2） subtitles=字幕（zi4 mu4）

subtitle=副标题 or 字幕

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.