I have to agree with Wonko. I get the impression you're smitten with the idea of a trip to Japan and chuffed at having been accepted by Temple.
You're a grown-assed man, it's (mostly) your money, and it is completely your life and you are free to do whatever you like...but I'm going to offer some unsolicited thoughts anyway. You may ignore every word of it without hurting my feelings the least little bit.
Let's pop the second balloon first: the program is just a summer revenue generator for Temple and getting "accepted" to it is no big deal. They'll accept whoever can fork over the cash. Don't let it go to your head.
At your stage of Japanese learning, it really doesn't matter where you are; being in Japan is going to convey no great educational benefit. You could make just as much real progress using Genki, doing the workbook, and having your fiancée drill drill drill drill drill the crap out of you.
If you weren't contemplating a move to Japan, I would think this a good opportunity to experience the place before you graduate and start on your life in the US. But since you're coming here anyway for an extended stay (don't marry her if you're not cool with the possibility of spending the rest of your life here), then it is a wasteful extravagance and an irresponsible self-indulgence. It doesn't matter how much of the bill gets footed by somebody else; the part of the tab you're left holding is still too much. If I had a very nice cigar box full of pretty shells I picked up at the beach and sprinkled with glitter, would you pay $18,000 for it? What if I told you somebody else would pay most of it and your cost would only be $5,000? That's essentially what you're looking at doing here in terms of lasting value and bang for your buck.
They're not going to give you any progress that you can't get on your own or any useful Japan experience running start that would justify the expense. If you're contemplating an international move, a marriage, setting up housekeeping, and paying back whatever student loans you already have out, then you're going to need every nickel you can scrounge as it is without throwing a buttload of cash you neither have not can spare at a university for them to teach you a few words and phrases that you are well set up to learn on your own anyway and which even if you weren't you could more reasonably do after you move here.
Frankly, the time and money would be far better spent acquiring useful certifications in your proposed field (IT) or getting some sort of TEFL certification to boost your employability.
As I said earlier, you may ignore every word of that without hurting my feelings. The only thing I would ask is that if you do manage to get the cash together and you do attend the program I would like to hear back from you a year or two after you've moved here if in hindsight the program was a benefit to you or if you wish you had put the money toward something of more immediate need.