The Japanese manage to hold their own against the Portuguese in the 16th century and instead of closing odff the country, deciode to become expansionist. Korea becomes increasingly difficult to defend, but Formosa, the Philippines, and parts of Indonesia are softer targets. By 1650, Japanese has become the lingua franca of trade in the area and as Japanese rule becomes entrenched, more and more people use it to communicate. Even after Dutch and British fleets break Japan's political hold on the area in the 19th century, millions of people use Japanese as the language of culture, diplomacy, and interethnic communication.
OJK, it's not quite the same thing as 'mother tongue', but it'll do for me. I's also define millions of Indians as 'native English speakers' by similar criteria.