Add description about translation target language

In the language settings page, if one of the listed languages is the target
language of translation, display a label below the language name.

Bug: 883964
Change-Id: Ie6604bebe6071cc67cf0a58cfb047b1d3c51f44a
Reviewed-on: https://chromium-review.googlesource.com/c/1426065
Commit-Queue: Yue Ru Sun <yrsun@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: anthonyvd <anthonyvd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629649}
9 files changed