To call "mostarda" a pickle is a bit like calling Barolo an alcoholic beverage. It really deserves to be called by its own name, with an explanation if necessary.

However, your decision will depend on the context and the degree of precision that is appropriate. Apple mostarda is a specialty of Mantua, and rather different from the mixed-fruit Cremona version. There are, of course, many other kinds of "mostarda".

Buon appetito,

Giles