Not Quite Written in English?

If you are using English-language records, is it possible that the writer slipped in a non-English word or a word in a non-English script? A native German speaker may have written in English only to occasionally slip in a German word out of habit? Or did a native Swede write a last name in his native script? That confusing word may be confusing because it’s not in English language or not in the English script.