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Signal is the currently state of the art from a UX and security point of view encrypted chat messaging app. The Open Whisper Systems team is behind the Signal Protocol, which was recently deployed on WhatsApp and currently being deployed in Messenger in order to enable seamless end to end encryption of the channel (voice and media included).
That said, the version 2.4 that is released for iOS contains the Greek translation
If you want to contribute with translating to your native language you can join the team on transifex which has a very convenient flow of translating all the strings and a nice built-in mechanism for reviews of the translations.