A reasonable amount of government debt is not a problem in a strong economy. If countries take on debt wisely and grow their economy paying the interest on that debt isn’t a problem. But as that debt grows as a portion of GDP risks grow.
Debt borrowed in other currencies is extremely risky, for substantial amounts. When things go bad they snowball. So if your economy suffers, your currency often suffers and then the repayment terms drastically become more difficult (you have to pay back the debt with your lower valued currency). And the economy was already suffering which is why the currency decreased and this makes it worse and they feed on each other and defaults have resulted in small economies over and over from this pattern.
If a government borrows in their currency they can always pay it back as the government can just print money. They may pay back money not worth very much but they can pay it back. Of course investors see this risk and depending on your economy and history demand high interest to compensate for this risk (of being paid back worthless currency). And so countries are tempted to borrow in another currency where rates are often much lower.
If you owe debts to other countries you have to pay that money outside the system. So it takes a certain percentage of production (GDP) and pays the benefit of that production to people in other countries. This is what has been going on in the USA for a long time (paying benefits to those holding our debt). Ironically the economic mess created by central banks and too-big-to-fail banks has resulted in a super low interest rate environment which is lousy for lenders and great for debtors (of which the USA and Japanese government are likely the 2 largest in the world).
The benefits to the USA and Japan government of super low interest rates is huge. It makes tolerating huge debt loads much easier. When interest rates rise it is going to create great problems for their economies if they haven’t grown their economies enough to reduce the debt to GDP levels (the USA is doing much better in this regard than Japan).
Japan has a much bigger debt problem than the USA in percentage terms. Nearly all their debt is owed to those in Japan so when it is paid it merely redistributes wealth (rather than losing it to those overseas). It is much better to redistribute wealth within your country than lose it to others (you can always change the laws to redistribute it again, if needed, as long as it is within your economy).