[youtube] How to download sub that is translated from a specific write sub or auto sub?

[wby238]

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Description of your issue, suggested solution and other information

I have search the issues, such as #4555 , it says that i can use like this youtube-dl '8YoUxe5ncPo' --write-auto-sub --sub-lang 'es,it',
so i use youtube-dl --write-sub --sub-lang zh-TW,zh-Hans --convert-subs ass https://www.youtube.com/watch?v=Vqs5mx9vs-M but it only download the zh-TW sub. What i want is the zh-Hans sub that is translated from zh-TW.
So i try youtube-dl --write-auto-sub --sub-lang zh-Hans --convert-subs ass https://www.youtube.com/watch?v=Vqs5mx9vs-M, and it successfully download the sub I want.
So how to download sub that is translated from a specific write sub or auto sub?

Here's the debug log:

youtube-dl --write-sub --sub-lang zh-TW,zh-Hans --convert-subs ass https://www.youtube.com/watch?v=Vqs5mx9vs-M
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['--write-sub', '--sub-lang', 'zh-TW,zh-Hans', '--con
vert-subs', 'ass', 'https://www.youtube.com/watch?v=Vqs5mx9vs-M', '-v']
[debug] Encodings: locale cp936, fs utf-8, out utf-8, pref cp936
[debug] youtube-dl version 2017.11.15
[debug] Python version 3.6.2 - Windows-7-6.1.7601-SP1
[debug] exe versions: ffmpeg 3.2.4, ffprobe 3.2.4, phantomjs 2.1.1
[debug] Proxy map: {}
[youtube] Vqs5mx9vs-M: Downloading webpage
[youtube] Vqs5mx9vs-M: Downloading video info webpage
[youtube] Vqs5mx9vs-M: Extracting video information
[youtube] Vqs5mx9vs-M: Downloading MPD manifest
WARNING: zh-Hans subtitles not available for Vqs5mx9vs-M
[debug] Default format spec: bestvideo+bestaudio/best
[info] Writing video subtitles to: ゆっくり妖夢と本当はこわいクトゥルフ神話Part3
 中文字幕-Vqs5mx9vs-M.zh-TW.vtt
WARNING: Requested formats are incompatible for merge and will be merged into mk
v.
[download] ゆっくり妖夢と本当はこわいクトゥルフ神話Part3 中文字幕-Vqs5mx9vs-M.mk
v has already been downloaded and merged
[ffmpeg] Converting subtitles
[debug] ffmpeg command line: ffmpeg -y -i "file:ゆっくり妖夢と本当はこわいクトゥ
ルフ神話Part3 中文字幕-Vqs5mx9vs-M.zh-TW.vtt" -f ass "file:ゆっくり妖夢と本当は
こわいクトゥルフ神話Part3 中文字幕-Vqs5mx9vs-M.zh-TW.ass"
Deleting original file ゆっくり妖夢と本当はこわいクトゥルフ神話Part3 中文字幕-Vq
s5mx9vs-M.zh-TW.vtt (pass -k to keep)

youtube-dl --write-auto-sub --sub-lang zh-Hans --convert-subs ass https://www.youtube.com/watch?v=Vqs5mx9vs-M
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['--write-auto-sub', '--sub-lang', 'zh-Hans', '--conv
ert-subs', 'ass', 'https://www.youtube.com/watch?v=Vqs5mx9vs-M', '-v']
[debug] Encodings: locale cp936, fs utf-8, out utf-8, pref cp936
[debug] youtube-dl version 2017.11.15
[debug] Python version 3.6.2 - Windows-7-6.1.7601-SP1
[debug] exe versions: ffmpeg 3.2.4, ffprobe 3.2.4, phantomjs 2.1.1
[debug] Proxy map: {}
[youtube] Vqs5mx9vs-M: Downloading webpage
[youtube] Vqs5mx9vs-M: Downloading video info webpage
[youtube] Vqs5mx9vs-M: Extracting video information
[youtube] Vqs5mx9vs-M: Looking for automatic captions
[youtube] Vqs5mx9vs-M: Downloading MPD manifest
[debug] Default format spec: bestvideo+bestaudio/best
[info] Writing video subtitles to: ゆっくり妖夢と本当はこわいクトゥルフ神話Part3
 中文字幕-Vqs5mx9vs-M.zh-Hans.vtt
WARNING: Requested formats are incompatible for merge and will be merged into mk
v.
[download] ゆっくり妖夢と本当はこわいクトゥルフ神話Part3 中文字幕-Vqs5mx9vs-M.mk
v has already been downloaded and merged
[ffmpeg] Converting subtitles
[debug] ffmpeg command line: ffmpeg -y -i "file:ゆっくり妖夢と本当はこわいクトゥ
ルフ神話Part3 中文字幕-Vqs5mx9vs-M.zh-Hans.vtt" -f ass "file:ゆっくり妖夢と本当
はこわいクトゥルフ神話Part3 中文字幕-Vqs5mx9vs-M.zh-Hans.ass"
Deleting original file ゆっくり妖夢と本当はこわいクトゥルフ神話Part3 中文字幕-Vq
s5mx9vs-M.zh-Hans.vtt (pass -k to keep)