I get that it's a (rather poor, it seems to me) pun, but you can't expect the OP to be able to tell a joke translation from an actual one. Not to mention that we have a rule that forbids misleading translations.
It wasn't I who dropped the "s", it was JK Rowling.
As Wikipedia puts it, "In the books and the associated film series, the names of the majority of these spells or the incantations used to effect them are derived from the classical languages, particularly Latin. These names are not grammatically correct in any language; most spoken phrases resemble Latin words of appropriate meaning but are not proper Latin themselves."
That being the case, I don't see why we are supposed to take them seriously. I grant that expecto patronum is valid Latin, but in the context of Rowling spells, that may just be coincidence. I find it difficult to see how you can have a misleading translation of a made-up language - ie one where the inventor hasn't published the vocabulary, rules of grammar and syntax. Unlike Esperanto. Or dare I say, Expectoranto?