There are probably less non dictionary words than dictionary words, perhaps better advice is to use words from a random language other than your own so the password cracker might make a wrong assumption and waste his time?
I think you may have missed the point that the XKCD cartoon was making. The question here is NOT how to generate passwords that are hard for computers to guess. That's a solved problem: simply generate a 128-bit string with a secure random number generator, and that's your secure password that computers can't guess easily.
The PROBLEM is to generate a password where the ratio of ease-for-humans-to-remember to ease-for-computers-to-guess is maximized. Using words from a language other than your own is NOT a good way to maximize this. Introducing just one more randomly-generated word from your own language will be far more effective by this criterion.
Just for completeness, your strategy assumes the attackers are assuming a particular language - which I would guess is probably the right thing for them to do, if they are looking for low-hanging fruit.
Like, say, every 3rd letter of the words you choose will be upper. hoRse or baTtery etc...
You've doubled the character set so it's not negligable.
There are infinitely many non dictionary words.
I also didn't say to use _strictly_ non dictionary words.