It really depends on how much information you want to put in your resignation letter. At least here in the US, it's a courtesy (unless there's some contractual obligation) to give 2 weeks notice before you leave, and not really advisable to give much more than that if you worry that it'll create a hostile work environment. I remember I gave way more notice at a job once and they started giving me really crappy tasks that no one wanted to do. They were just trying to squeeze everything out of me before I left, and that has since made me much more cautious about sharing my plans at work.
I can understand the desire to explain yourself, and if you feel like you've benefitted from your employment, to thank them for that, but it's not actually necessary. In my experience, that desire to "do the right thing" as a leaving employee is the sign of a young or inexperienced worker who hasn't been screwed over by their employers much yet, or worry about being able to use the former employer as a future reference. Understand that everything you've been taught at this job has been ultimately for the company's benefit, and whatever you've gained from the experience is kind of inconsequential to their bottom line. I've even had employers who refused my requests for specific training, because rather than realize that keeping me engaged and learning is a good way to retain a passionate employee, they were worried that once they paid for me to level up I would leave. So I did the training on my own, stuck around just long enough to pay it off while phoning in the job (why should I care if they clearly don't?), and left anyways. I have submitted notice that simply reads "As of X date, I tender my resignation from this position. Thank you." You aren't really required to put anything else in, you're just making a paper record of your decision to quit so they won't mess with you. I've also had an employer act confused and squeeze more time out of me because I didn't put it into writing. Capture EVERYTHING in writing and e-mail; no one else will cover your *** for you.
If you really want to explain, you can say something like "It's been a real pleasure to work with you and I'm grateful for the experience and knowledge I've gained from my time here. I've been given a chance to fulfill a lifelong dream to move to Japan and study the language that I can't afford to miss. As difficult as it is, as of ##/##/#### date I must tender my resignation from my current position with X company in order to pursue my dream and personal development. I hope to remain in touch as we cheer for our mutual growth and success."
But really, outside of legal/contractual requirements, it's all up to you.
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