Internationally accepted currency is the $ and post that comes a few others like the yen, Euro etc.

However, for every country there are a few currencies that a country trades in. this number is highly skewed but trade can also happen if the importing country is also trading in it. But that is extremely specific.

For ex, Chinese yuan is an internationally traded currency but India has no direct market for it and hence it is not traded in the country. the transaction thus may happen in $ or Euro with a Chinese counterpart.

But in summary, $ is the international currency for any trade and hence it is important for transactions. A country that has sanctions like N.Korea, Iran face problems in securing $ as the other countries and companies cant directly invest in them (which is a source of $ or else exports) and hence they are unable to either pay or recieve $. Thus their economy goes downward. This in a way earlier used to be dollar diplomacy but currently one can call it as $ bullying.