According to the U.S. Treasury here is a list of the highest holders in volume of U.S. Debt:
It is not surprising that the two largest nations here have been attempting to lower their currency value vigorously as to increase their debt value in local currency terms. Japan has since the introduction of “Abenomics” in late 2012 reduced the value of the Japanese Yen from 78 $USDJPY to around 119. Meaning that the debt value in Yen has increased by well over 45%. No a bad investment.
For Forex traders there are clearly vast opportunities to play on the intentions of the central and reserve banks here as there is a strong need to have a strong U.S. Dollar for there countries.