Japan has extremely high levels of savings and nearly all of...

Japan has extremely high levels of savings and nearly all of that public debt is domestic, some of which is also inter government lending. The very low interest rates on the debt mean that the repayments are not much of a burden.

I haven't looked but I think the US figures also need to be adjusted for their recent bailouts, which will push them further up the ladder. Future bailouts and the potential for a $2 trillion bad bank and forecast trillion dollar deficits will see them rise even further (as will other countries). The US also is the largest foreign debtor nation by quantum and this will also get larger (although as a percentage of GDP there are worse nations at present).

Cheers
SBC