if there is more than one subtitle for the language, use the first one

When there is more than one subtitle for the language, use the first one.

Currently, when I'm watching the google io, i found that the subtitle is not right, but it's ok in the youtube. After some debug, I found that some language has more than one subtitle, youtube use the first one, but the `youtube-dl` use the last one.

This patch use the first subtitle for the language. However, it may be a better idea to add the `name` to the subtitle.

For Google IO 2014's Keynote, https://www.youtube.com/watch?v=wtLJPvx7-ys, there are two subtitles for english, `<no name>` and `cc1`. As the youtube use the first subtitle for english, so it's ok, but the `youtube-dl` use the second one, so it's wrong.