I just finished playing through your demo, and I think I understand why people are having trouble expressing what's "off" about the English version. There are a few places where the word choice feels wrong or is incorrect (like "he is probably a friendly guy with a wide range of communication" - communication isn't the right word here, for example), and a few small errors in things like pluralization or minor punctuation. But a lot of it is more subtle. The word choice and sentence structure aren't necessarily incorrect, but the sentences aren't phrased a way a native speaker would say or write them. Toward the very beginning of the game, the MC says "Loneliness perhaps is something that does not exist in my heart since I still have many adorable and cheerful friends around." It's not wrong, but it sounds, well, like a sentence that's been translated from another language.
I think having a native English speaker work with your translator would help a lot. These sorts of problems can be some of the hardest to catch when you're working in a second language, but will make a big difference in the playability of the game for English speakers.
You mentioned that you plan to revamp your GUI, but I figure it's worth pointing out a few things anyway. First, there's currently no way to go back to the main menu without quitting the game, which I'd like to see implemented. I like the idea of having a log to go back and read previous lines, but it only shows dialogue, not any of the narration, so if you click through too fast, you can't go back and read what you missed. Also, more save slots please
All that said, I think your art is lovely and I'm excited to see where this story goes. Hopefully this was helpful! Best of luck with your kickstarter!