Year abroad in a country with low coursework standards: Am I sabotaging my future career?
I'm an assistant professor data mining in another EU country.

Suppose that you choose to do this exchange semester at Keio University, and then apply for a PhD position in my lab. What your resume would tell me, is not that you elected to go to a university with relatively simple courses*. Instead, what it would tell me, is that you elected to acquire a double degree, with exchange semesters in two countries on this planet that are both vastly different from your home country and vastly different from each other. That is far from taking the easy road. Instead, you show me that you are willing to go the extra mile, in order to develop your skills more broadly.

This broad development is very important for a prospective PhD student. During your PhD, technical mastery of statistics and data analysis is only part of the required skills: you will need to collaborate with others in the group, clearly communicate your results in written papers and oral presentations, and manage your own professional and personal responsibilities. Going on exchange semesters gives you an advantage on your application. My data mining group encompasses about 50 people, from 18 distinct nationalities; an interest in and experience with a rich variety of cultures is worth much more to me than a perceived relative difficulty of the exact courses that you follow.

Good candidates for PhD positions are scarce. A broad development sets you apart from other candidates, if you choose to apply for a PhD position.  If I were you, I would take the experience of going to Japan for a semester. That this aligns well with your personal life is a nice bonus.

*assuming that this simplicity is actually the case, of which I have no knowledge.