This isn't necessarily incorrect English - it's just not great English. If it was Hatsuharu saying it, it might not look as poorly.
1. Shiratsuyu's English is definitely incorrect. In fact, reading it at face value (ignoring the improper order of the "not"), she's asking if they had been introduced to the mochi, not if they knew where it went.
2. "Poorly" is an adverb, but you should be using an adjective such as "poor" or maybe something like "strange", which means you're ironically using incorrect English to argue something is correct English.
3. It isn't Hatsuharu saying it or anyone else who should speak in an unusual way. However, except for Roma, they speak in a way no native English speaker would speak, to the point that it's distractingly obvious. Those characters that use some form of dialect have them for a reason, to make them sound different because it makes their characters distinct and maybe even says something about who they are and what their characters are about. Having odd, stilted language that distorts how readers view what the character is saying in ways you as an author don't intend is poor writing even if you technically follow the rules of grammar, and that still warrants being labeled "Engrish".
