It depends what currency country B is owing to country A too, if
you owe in country A's currency, then there will be problems,
because you can't control the monetary policies in China. The more
US dollar you convert to RMB, the more the RMB is going to
appreciate, because the money supply in China is more or less fixed
(ie assuming that China has a free floating exchange rate regime,
but.....).
But the debt due to China is denominated in US dollars, the US
Treasury can print as much as they like (heard on some documentary
that they can churn out billions of US dollars a day), sooner or
later the world is going to realise that US dollars are nothing
more than banana leaves. 
you are making it more complicated by bringing in exchange
rates.
But since you bring it up, one of the possible consequences of
this 'saving' of stupid banks is inflation.
Since govts will start printing more money to pay off their
debts, more of their currencies start mounting in the market,
resulting in inflation.
