This app was mentioned in 2 comments, with an average of 0.50 upvotes
Matrix is an open protocol for decentralized communication - offering a slack-style communication/communcation service. The differentiator? It's all open source and free; you can run your own server, and you can encrypt the data end-2-end.
Check out a client like Vector - and if you are interested to learn more, see how you can get involved!
Or check out the code directly:
I've created a chat room on the open source matrix network for anyone to join. Can download an app directly here: http://matrix.org/console/download/ and more info here: http://matrix.org/blog/try-matrix-now/ ... There's also a great browser client to use: http://vector.im ... The room is #japanese:matrix.org
The android client is also in the playstore: https://play.google.com/store/apps/details?id=org.matrix.androidsdk.alpha
URL straight to the room using the vector client: https://vector.im/beta/#/room/#japanese:matrix.org