Think they meant what made the dialogue boxes appear empty except for one or two words when the slider was all the way up for English. You had mentioned something about compound words in Japanese that made sense but had no proper words in English to translate into right?
Oh that! Okay, I mean, I think that even happens in Japanese as well. I'll try to use my own knowledge and assume how the dev approached text boxes. The amount of time the text boxes stay up probably goes by the amount of characters (letters) in the text box. Keeping that in mind, some the of the dev's English translations are less than 5 letters. If those translations appeared in the text box by themself, the textbox probably wouldn't stay up long. I'd like to believe the dev coded a minimum amount of time for the text box to show up (like, stay up for at least 3 seconds) but it doesn't seem that way. It also could be bugged too. There might also be things that force the text box to hide or go away. I can't confirm or deny this.
What I'm thinking is happening when using the dev's English MTL is that either you're getting really short text or you're getting empty text. There definitely was English text that was left blank and the game was probably pulling that as the only phrase when it formed the sentence. Thus, leading to an empty text box that appears briefly.
I haven't noticed any of that happening with my patch, as I restored many of the blank MTL translations and removed/edited a lot of the short ones.
Basically, the text box issue boils down to blank translations, very short words, how the game combines phrases to make sentences, and the game generating sentences using those blank translations and very short words. Anything other than that is probably a bug or something forcing the text box to show/hide or change the text in it.
The game is pretty easy to break if you're really trying to break it tbh. The UI is especially prone to it because of how the dev approached things. When you click an action, it feels like it "queues" up. The UI doesn't lock when you select something most of the time, meaning you can spam other buttons in succession, potentially putting the game in a weird state. For example, you can sometimes soft-lock the exposure part if you spam click the "Fall" button which may lock your partner into the fall pose forcing you to restart since no other actions will work. You can potentially also put the game in a bad state by spam clicking stuff while other things are happening as well. I could list like 20 other bugs, but casually playing, you won't see most of them and they're mostly minor issues. I'd feel bad flooding the dev with semi-mtl bug reports because a lot of them are minor. The only reason I reported the black screen bug was because it was game-breaking.