It's interesting that a low occurrence of unique words in a story can be an indication of a lie. Does that position people speaking a secondary language in an unavoidable place of distrust? Beyond body language, it's just more difficult to tell if someone's story lacks richness because they're not telling the truth, or they're simply not comfortable with the language and have a small vocabulary to draw upon.
I think ideally you'd compare the story you're investigating with other, provably-truthful stories told by the secondary-language speaker. That way word reuse could be a relative value rather than an absolute one.
