I've been reading up on Diceware recently, and I'm wondering if using a mix of languages would make it even more secure. On the face of it, it would force a potential attacker to consider an even larger search space, even if they know that you speak e.g. English and French. Using even more languages would increase the search space further.
The main issue I can see is usability. Would this be harder to remember than a monolingual passphrase, and would the difficulty of remembering your passphrase depend on your level of fluency in that language?