I'm also stuck on it. I'd been "there" before. But he had solved by doing several random tests.I had used a Shanee's sierra_v4.dmg. Today, not even one solution dmg gives me a positive result.
I always have a perfect boot, but it stuck me there in the list of languages.
I have this same problem in the Sierra and High Sierra.
I think it has some connection with the way they create the installer. When I accompanied them to create the image I noticed that they did not use the official installer of the High Sierra. They use the same (faulty) installer they created for the Sierra.
I know the Shanee V4 version worked then and I had used SierraZone just to boot and use the Shanee image.