It's optional for hosts, you turn it on via the broadcaster dashboard or options -- and it works on mobile too. The thing is, even if you choose the channel's language as your own you can still chat in whatever language you want, which robs the feature of a lot of its potential power; it's basically like an age gate on a porn site in its current state. Twitch explained the reasoning to us as such:
Broadcaster Language Mode is an opportunity for foreign language broadcasters to create communities inclusive of their native language for a more cohesive chat experience. This helps create vibrant foreign language communities that can easily engage with one another.
It's up to the broadcaster and their moderators to decide how exclusive their channel is. Therefore, all messages will be sent since we don't moderate for what language is being used in a chat with Broadcaster Language Mode enabled. A broadcaster may like that they have viewers with a different language participating in chat, or they may have viewers who use English colloquialisms. We don't want to force any of that out on our side.
If Twitch could promote chat questions pertaining to Lightbar support, then we'd really be in business.
[Image credit: camknows/Flickr]