What Should I Look For When Assessing a Bilingual (Japanese-English) Child’s Language?

Hi there, I am a Speech-Language Pathologist assessing a bilingual child's speech and language development and I want to be as respectful as possible of language differences and cultural-linguistic influence on the child's English.

I have accessed so many resources online but this is a unique situation that isn't talked about often. Now, I actually know a bit about Japanese from teaching English there many years ago, but I'm looking for better input from those who truly know more about both languages, especially informal or cultural influences. Because I am assessing their speech and language, every tiny error is looked at and I want to make sure I cover as many bases as possible so the child doesn't get labeled with a language disorder when really it's a language difference!

So, I'd really appreciate if you can give me ANY and ALL advice you can think of to help me identify why their errors in English (such as sentence structure, word endings, lack of context in conversation, etc.) might be influenced by Japanese language input. In addition to errors in things like verb conjugation and countable vs. uncountable English nouns, they also say things like "probably", "maybe", "but", "I think" and "kinda" and repeat themselves a lot, and overall are hard to understand not because of their speech sounds, but because the vocabulary and syntax they use confuses most monolingual speakers. Their conversation is, for lack of a better word, jumbled.

I really simplified what I'm looking for here but…truly, anything you can think of, anything you have noticed bilingual learners do when their Japanese influences their English, I'm open to all suggestions! Bonus points if you have studied linguistics and can describe these concepts to me in skilled terms (Phonology, Morphology/Syntax, Semantics, Pragmatics). Extra bonus points if you're a fluent bilingual speaker that was raised in the American school system! I'd love to hear your perspectives on growing up here so I can better understand what my student is going through.