Why do Kayoko Ann Patterson and Rando Yaguchi lapse into English?

Is there an in-universe reason why the characters occasionally speak English to each other?

From a quick re-watch it seems to me that Kayoco is the one who slips into English on occasion and, consequently, Yaguchi responds in the same language, as politeness would dictate.

Kayoco is intended to be fully fluent in English as she is only of Japanese heritage...she refers to Japan as her..

Maternal grandmother's homeland.

Unfortunately, the actress (Satomi Ishihara) does not come across as fully American as her English is heavily Japanese accented ^*.

^{* My phrasing might not be quite right here and the actresses language skills far exceed my own so any criticism is based on the movie role only.}

Indeed, Wikipedia states...

She became excited after accepting the role but stressed that her character features "English-heavy dialogue", stating, "Sometimes it's so frustrating, I just want to cry."

So, why does Kayoco slip into English on occasion...I suspect to emphasise the fact that she is American and would lapse into her (movie) native language.

This would be especially true in times of high stress or excitement..or, perhaps, when Japanese just doesn't have the same expression

[Note: I do not know Japanese but I'd imagine that there are cultural references that do not translate easily.]

Yes, as has been stated above, she is supposed to be American. However, Satomi Ishihara ISN'T in real life and speaks with a distinctly non-American accent.

And while flipping between English and Japanese in conversation (something akin to Spanglish from where I'm from in South Texas) in Japan it's not unheard-of in film or on the streets. I've heard it fairly often from Japanese Nationals (JNs) while living there. It can sometimes sound quite unnatural if the subject is not a native English-speaker, as I felt was the case in Shin Gojira.

Too bad really, her jarring, heavily accented dialogue was what threw me out of the movie everytime she opened her mouth and slipped into "American." I winced a lot more than I'd've liked. Yet, I applaud her for her effort, though. It took me many, many years to shed my own American-accented Japanese and somewhat master a more Kanto sounding dialect.