I'll be honest here, I just consider it a continuity error and pretty much ignore it! That's fine by me, but I probably can't give you an answer you'd find satisfying.
Now I recognize it's an especially big error, but that stuff happens quite often in media, Twin Peaks included. And in Jeffries' case it's a date which I actually do find a whole lot easier to ignore because I really don't care much about timelines not matching up in this kind of work... I barely know the dates myself. That's just not how I approach Twin Peaks.
Of course I know it's a mistake, but for me personally it's one I find it easy enough to explain away anyway, yeah. I don't even view it as a prediction, just as Jeffries being in a very confused state and getting it wrong, but still recognizing something's up with the date. That's clearly not the intention but it works for me.
Also, if I really can't reconcile something I'd just say that one line of dialogue isn't canon, I'd never go and dismiss the rest of it. That's just not how I approach those things. And I'd especially never wait for a reference in season 3 to say, Leland teaching some Norwegian. There are absolutely no issues there and it's a lovely scene, the waiting for acknowledgment approach isn't really something I get, that's definitely not me.
That make sense to you? :s