You can pursue an economic policy that creates high levels of inflation without necessarily triggering hyperinflation. The goal of course would be to gradually inflate away the value of the debt, not to pay it all back immediately.

Wouldn't that wipe out the very same people who invested in Japan? There are a lot of regular Japanese people who hold Japanese debt. Wiping it out via inflation is equivalent to wiping out the wealth of regular Japanese people to save the Japanese government.