In a preview in the german PC Games 06/2017 magazine, the release date for Elex has been mentioned. Until now it was only known that Piranha Bytes newest RPG will be released in the 3rd quarter this year, but according to the preview, it will be released in August. The exact date isn't yet known however, but a release close to Gamescom (22nd - 26th August) seems reasonable. The August release will be almost exactly 3 years after Piranha Bytes' last game, Risen 3: Titan Lords.
Update 31st May: In a news the PC Games has again confirmed August as the month of release.
Update 1st June: That was an error, the release is on 17th October 2017 instead!