In a move to give more control to casters over the stream chat panel, Twitch have introduced Broadcaster Language Mode. This allows casters to set a default language on their channel page and viewers will need to confirm they can speak that language in order to post in the chat panel. If your browser language preferences are set to the chosen language by default you won’t be prompted to confirm you speak that language.
This change appears to be an attempt to curb abusive chatter in a language the caster or moderators do not understand and therefore can’t moderate.
A lot of non-English speakers also speak English and in fact broadcast in English but are equally capable and willing to interact with their viewers in their native language. A better solution would be the ability to set a number of acceptable languages for your channel.
Perhaps Twitch will respond to user feedback and expand on this idea, although I don’t know how well or speedily they respond to user input.