According to Halacha, the Japanese are not Jewish. Not converting them does not delay redemption.

The Torah has promised that at the end of exile, we will do Teshuvah, and immediately we will be redeemed. Utilize your energy and capabilities to bring people to practical mitzvot. As all Jewish people are one structure, ones Torah, and mitzvot help all Yidden. Especially when increasing the study of the soul of Torah. https://www.torahrecordings.com/rebbe/igroskodesh/012/006/4154b