Reading through the dev's
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using DeepL and my very limited knowledge of Japanese it seems that it's not the game engine being complex that's the problem, it's him being uncomfortable with sharing the inner workings / source code of the engine because he says it's needed in order to be able to translate the game. He's currently looking for a translator who's willing to work with him like this:
He'll send the text files over to a translator and then the translator sends the localized text back to him and he'll add it to the game. Kinda wonky but people have done this before in the past and it's worked reasonably well.
People are actually willing to translate his game, he said in one of the articles that he's gotten offers from various people on ci-en and twitter and other places but right now he's trying to decide if he'll go with a publisher, AI translation or going solo with the help of English volunteers. He said he initially wanted to use DLsite's translation services but DLsite has temporarily taken down the service for some reason. The decision on how to localize the game is taking a while because of real life stuff he's sorting through right now.