Just tried this (2018-4-29) Work for me!
Thanks for building this!
(I am just trying different plugin to get familiar with Discourse plugin ecosystem.)
0. Install
cd discourse/plugin
git clone https://gitlab.com/majalorg/discourse-available-locales
restart rails s
1. Set available language (as admin) (Chrome)
2. Use another browser to login as normal user (Firefox)
Thanks!