Signal is an encrypted chat messaging app that is considered the state of the art from both a UX and security point of view. The Signal Protocol, developed by the Open Whisper Systems team, enables seamless end-to-end encryption of channels, including voice and media. The protocol has been recently deployed on WhatsApp and is currently being deployed in Messenger.
As of version 2.4, Signal for iOS supports Greek as a language. That’s a small contribution I made, since I was surprised when I noticed that my favorite chat app did not support Greek as a language. The process was straightforward and the Signal team was very responsive.