...Because an interational custody battle is going to be a cakewalk, right?

Okay, she doesn't say here if this was a constant point in their marriage, or something that her husband only mentioned once or twice after she made the initial promise. If he leapt out of bed each and every day saying, "Only 694 days until the move!" that's one thing. If they're four years and five months into their marriage, and he said, "Hey, you once said that after we were married five years, we'd move to Germany. Time to start house-hunting!" that's something else. The truth is probably somewhere between those extremes, and really, it's not a competition or a battle. It's a BIG DEAL, it will have a huge effect on their lives (positive or negative), and just because they compromised at the beginning of their relationship shouldn't mean that they can't make a more adult compromise now.

Like I said, things have clearly changed from their initial expectations of life together. Doesn't mean they don't love each other deeply, or that she shouldn't follow through on her end of the bargain. They need to take a little time and make an actual plan. She needs to learn the language, research work opportunities (at no point in her letter does she say that he'll be 100% financially supporting her), TRAVEL there, and get excited about this. Right now, she probably feels like she's being dragged there against her will.