YouTube captions (subtitles) can now be auto-translated: only after the video has started playing, on the bottom right of a video click on "CC" to turn it on, then, next to it there will be an arrow which if clicked, pops up the translation menu (kind of hard to find, i know). Here are some pictures, so you can understand better:
The languages available are the ones also used in Google Translate. But of course, the translation can make you understand what's being said, but that's about it, as automated translation has it's limitations.
Captions usually are turned off by default so if you want to enable them by default in an embedded video, add &cc_load_policy=1 at the end of each URL in the embedded code.
Here is an example:
[credits: Matt Cutts]