As this is the time when we're all coming up with wacky mottos, I thought I'd try my hand at our department's private motto.
Having run a few variants through Google Translate, and coming out with a Latin word salad, I thought I'd try my old Latin dictionary. It still comes out as a poor Anglophone substitution rather than a witty slogan.
"We overpromise, (and) we under-perform"
Overpromise, as in "to promise the earth". To claim that we can and will do more than your "wildest dreams could imagine". That's the first idiomatic sticking point.
Under-perform, as in lacklustre, shoddy or slapdash. I'm sure there's a pretty way of saying that, other than "sub-optimal".
And then how to stick them together in a way that doesn't hurt to look at?