It is a good point about concentrating assets solely in yen, or any single currency. I feel much happier with a broad based allocation of yen, dollars and sterling for my cash and bonds.
For equities it’s not so much of an issue as most developed markets have a broad based suite of corporates with diverse revenue exposure. Even if you buy a Japanese equity fund it’s full of Toyota, Sony, Hitachi, Tokyo Electron etc. Even the megabanks earn 30%+ of their profits overseas.
Diversifying cash, currency and bonds into a few currencies is a pretty good idea.