Roughly the same as Argentina but accumulated over a much larger time period.
Of course as a nominal figure I suspect American debt is larger (eg. in name or denomination value, not as a percentage of GDP).
However this will only be a problem for Argentina, as America and Japan are sovereign currencies, mostly in debt to themselves. The main risk of their position is lending downgrades, and buying power downgrades, but they probably won’t matter all that much in any defacto sense.