I feel like I see this kind of thing a lot in sf stories. Made-up example:
“Feh!” she said, in perfect unaccented fluent Old High Garzingian.
I feel like a single interjection, or any other single word, or even a brief phrase, isn’t really enough of a sample for an observer to be able to tell whether someone is a fluent speaker of a given language.
I know this is a tiny issue. But it always throws me out of the story, and I feel like I’ve been running into it fairly often lately.