"overseas people" isn't used
"people overseas" would refer to people in different countries, without reference to their nationality. You could say "Japanese people overseas" to specify Japanese natives living abroad.
"foreigners" is a non-specific term, "foreign" is determined by context. "Foreigners in Japan" would refer to non-Japanese living in Japan.
Considering your example, I would say either "people who aren't Japanese" or "non-Japanese..." but as Wonko said, it's better to avoid generalities like that.
Natto is a fermented food popular in Japan, but it is an acquired taste and many people don't like it, even Japanese