The simple solution to that is to just allow any spelling of a word that is considered "correct" by dictionaries. Using your example, both center and centre could be considered valid answers when given a word pronounced like /ˈsen(t)ər/.Julian_Kaiser wrote:
suppose Yyotta picks up a word.
US participant: "center"
anyone from outside US: "centre"
In such cases, the US participant would be marked wrong for their altered way of spelling.