Thanks for checking! No, I didn't mean an awkward literal translation -- that's why I suggested a gloss. I think I've seen glosses or explanations in lieu of translations in other WordReference entries on occasion. That said, I am not a lexicographer! I just wanted to flag this entry for the WR team's expertise.
Perhaps the English side could say something like: "Spanish spelling mnemonic: m comes before p and b"?
And it may also help to add a note or notes, depending on what the system can display:
If the note is associated with the Spanish expression, I would expect it to be in Spanish (and nothing at all about English), explaining that this rhyme is a spelling rule, etc.
If a note can be associated with the English translation, it might make sense to say something like "Note: For English, the saying "i before e except after c" is a comparable rhymed spelling mnemonic."
Side note: for the English saying about I/E, I wouldn't like to call it a "rule" in a dictionary entry, lest learners think it's absolute/universally true. But not only is there a second half to the rhyme, probably too long to print ("... and when sounded as A as in neighbor and sleigh"), there are also many common exceptions. If "mnemonic" is too technical, I guess I'd call this sort of saying a "rule of thumb."