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Just Retrospective so far. We plan to do Reborn as well but it takes time.

Hope you enjoy!

你可以详细说明一下你的问题吗？

The beta for 3.7 will be coming out soon, the release will follow a few weeks after.

Not all the main story has been adapted yet. 
We are approaching the end of part 1 which covers the whole Retrospective storyline. There are 3 parts total in GGZ.

Development is still happening yes, it just takes time.

If you want to help out feel free to pop into the Hoyostans discord. Were an open community so feel free to join and contribute to things in your own way, or just hang out and talk.

Reborn most likely won't be coming out until next year, and when it starts it will take a while for a lot of the story to come out as its very long. I would say start Retro now, then if you want to read Reborn when it releases go ahead. The stories are different eras so while Reborn does have some overlap with Retro since it comes after it in the timeline, you can start with either.

There is no correct order to play the games. Each contain their own independent story (except Honkai: Star Rail which is a sequel to Honkai Impact 3rd set in the rest of the universe away from Earth). While the games do share a connected universe and many concepts which build upon each other to give a bigger picture and make things more clear, each story stands up on their own and can be played in any order. Guns Girl Z has been receiving updates alongside Honkai Impact 3rd since HI3 released and the story is still going. Zombiegal Kawai/Houkai Gakuen 1 was Hoyo's original take on the genre, though due to a variety of factors they chose to make it into a gacha game and release it as GGZ.
Currently Operation Gekkou is approaching the end of GGZ part 1 (Retrospective). GGZ currently is on its 3rd part so we still have a large amount to do and the amount keeps growing.
If you would like my opionin on where to start, I would say to either start with HI3 or GGZ. Since HI3 has the more complete story available in multiple languages that is probably the best option. 

Not yet no, there are still many missing parts of the story for that phase which we are still researching and putting together. Phase 5 will be revisited once Phase 6 is completed to prevent its incomplete nature from holding back the project from getting to the end of the story.