It's illegal under US law. 18 U.S. Code § 478 says:
Whoever, within the United States, with intent to defraud, falsely makes, alters, forges, or counterfeits any bond, certificate, obligation, or other security of any foreign government, purporting to be or in imitation of any such security issued under the authority of such foreign government, or any treasury note, bill, or promise to pay, lawfully issued by such foreign government and intended to circulate as money, shall be fined under this title or imprisoned not more than 20 years, or both.
There are several similar laws for similar crimes:
§ 479 - Uttering counterfeit foreign obligations or securities
§ 480 - Possessing counterfeit foreign obligations or securities
§ 481 - Plates, stones, or analog, digital, or electronic images for counterfeiting foreign obligations or securities
§ 482 - Foreign bank notes
§ 483 - Uttering counterfeit foreign bank notes
§ 488 - Making or possessing counterfeit dies for foreign coins
§ 502 - Postage and revenue stamps of foreign governments
According to version of the South Sudan penal code I was able to find, counterfeiting is illegal. But if I understand section 7 correctly (which I might not), most counterfeiting is not prosecutable under South Sudan law if it is not done in South Sudan. Having a counterfeit revenue stamp, however, would be, as would fraudulently altering a coin. Given that there's not an extradition treaty and that it's already illegal under US law, this probably doesn't matter much, though