Is expiation obligatory upon one who had intercourse with his wife during the daytime in Ramadan, believing that day to be Eid al-Fitr, based on another country's announcement of Eid, only for it to become clear that the day was still part of Ramadan in his own country?

You must make up for the day you broke your fast, and expiation (kaffarah (definition)) is not obligatory for you, because you did not intentionally violate the sanctity of the month. Expiation is only obligatory for the one who acts intentionally, but it is not binding upon the one who makes a mistake, due to the Almighty’s saying: ﴿Our Lord, do not impose blame upon us if we forget or make a mistake﴾ and His saying: ﴿And there is no blame upon you for that in which you have erred, but [only for] what your hearts have intended.﴾