When an Amara video page is created from a YouTube video that is already subtitled , that Amara page does stream both the YT video and the YT subtitles. However at least in some cases (1), the video language is not indicated in the resulting Amara video page.
Considering that the option to connect one's YT channel to Amara automatically creates an Amara video page for each of the channel's videos, including already subtitled ones, and that the same thing happens when anyone adds all the videos of anyone's YouTube channel via the "Or Submit Multiple Videos" button in the Subtitle Videos page:
(1) See e.g. attached screenshot for subtitle creation as it appears in "Sign educators on Mandela interpreter CNN com Video - copy for captioning" http://www.amara.org/en/videos/B4Xz6UUnbaDf/info/ presently. Sure enough, should someone then translate the English subtitles or upload an existing translation, s/he'd have to add the video language indication, but often, the original subtitles don't get translated.
(2) Taking again the example of the Amara video page mentioned in (1), if you search the Watch page for 'sign educators' in videos in any language, you find it, but if you do the same search for videos in English (a logical thing to do if you don't know that the video language is undetermined), it doesn't appear.