There was a lot of Japanese immigration to Brazil, and recently there has been a movement of their descendants "returning" to Japan. These descendants are 2nd-4th generation Brazilians and often only speak Portuguese, resulting in the signs you mention in Japan.

I always imagined it was related to Portuguese adventures to Formosa and perhaps beyond, to Japan. But I did have lunch at a Japanese-Brasilian's house this weekend...

Some neighborhoods around Boston had a Portuguese option at their ATMs. Got ripped out and replaced with Spanish when the banks upgraded their software at some point, but since the neighborhoods were still heavily Brazilian that didn’t make sense.