The key point is not the Debt/GDP ratio by itself but the sustainability of the debt over time.
The government borrows money from the market and uses taxes to repay it, as households borrow money from banks and use their income to repay it.
Before moving on, let's look at a simple debt accumulation equation
where b(t+1) is the debt/GDP ratio in period t+1. It depends in s, that is the primary surplus of the government, r, that is the real interest rate, and gamma, the growth rate of the economy. In Japan the debt/GDP ratio is high but they also have very low real interest rates and a primary surplus which allow them to repay the debt. On the contrary Greece, accumulated to much debt enjoying the low interest rates granted by the Euro. When the crisis hit, markets lost confidence on Greece's ability to pay back. Thus they sell off their bonds. Interest rates sky-rocketed and because of the economic crisis the growth felt down. Moreover, the government had, and still have, many problems in collecting taxes. Therefore, Greek debt is more unstable and dangerous than the Japanese one, even if it is lower both in relative and absolute terms.
In a nutshell, default arises when you run out of money for repaying interests regardless of the amount of the outstanding debt.
Nonetheless, Japan is in trouble. The more the debt growths, the more budget resources will be allocated to repay interests. Hence, forcing the government either to increase taxes or reduce spending in other areas, something that is politically difficult. Anyway I suggest you to watch this video, it gives a roughly idea of what is going on there
Japan's Debt Problem Visualized
Regarding your question on domestic vs international creditors, they are important when the government wants to decide whether to default or not. The more debt is owned by resident the more costly is going to be to default as your citizens would bare the loss. And, in any case, the government has to pay back interest regardless of the nationality of the creditor.