Coming from anyone else, Sherlock supposes, that might be an insult. It's a common one. He does too much, is too involved. It makes him freakish, makes him strange, irritating, unlikeable. John doesn't mean it quite that way, though, it seems.

“Well, you have to get the words right, don't you? Something just for that, something you wouldn't say otherwise.” Something whose meaning is tied to the spell and to the spell alone. A dead language suits that purpose perfectly.

“Also a bit stupid to run about shouting things in English to do magic, isn't it? Sounds daft, I mean.” Also not ideal. Too much risk of semantic entanglement. Hard to focus on one thing when a word can conjure up so many others.