It's sorta like how the Klingon characters' dialogue is in English most of the time, except for random things like them mentioning how many kellicams away something is.
Well, what's wrong with that? A kellicam is a distinct unit of measurement, no doubt different in length from a kilometer. 500 kellicams would be a different distance from 500 km, just as 500 km would be different from 500 miles. It's not just an arbitrary substitution of a foreign term for the same thing; it's the correct name for the specific unit of measurement being used.
I mean, if I say that I'm reading 500 roentgens of radiation, that's not a gratuitous insertion of a German word, because that's the correct and only name for that particular unit of measurement.
What's wrong with it is that a roentgen is a real thing that I can look up. Not even Memory Alpha is sure what a kellicam actually is.
That said, one of my favorite (bad) sf devices is the fictional space measurement-- I love it when characters start measuring distance in "spatials", or time in "spans", or what have you