There's some debate about where the English phrase "out of sorts" comes from; since the individual characters of a typeface were called a "sort", it's feasible that "out of sorts" originally meant to run out of a needed "sort" for typesetting. A common method of dealing with that would be to employ a different "sort" in a different typeface - which would look like your zwiebelfisch.
It's not likely that anyone will know what you mean using that phrase though, as it basically means "not orderly" at this point. Since we no longer deal with manual typesetting, we'd probably just call it a "missing glyph" or "wrong font" at this point.
Thinking about this a bit more, I think "odd sort" is likely the closest English translation. It describes a member of a set (a sort) that is unlike the others (odd). A common expression would be eg., "he's an odd sort of fellow" meaning he's different than your average person. Given "sort" history with typesetting, it's possible (though I have no references) that it originally described the exact scenario zwiebelfisch does. Though it has mostly lost any historical connection (if it, indeed, ever had any) it would still adequately describe a character printed differently than others around it.
"Odd fish" is another English term that may have some historical connection. An "odd fish" is a name for something that's of an "odd sort" (eg., he's an odd fish) and it would seem to share the same "fish" connection with zwiebelfisch. I'm not aware of English ever using "fish" as typesetting slang, but serif in Japanese is uroko - fish scales so perhaps there is a connection there.
