The family is preparing to leave for a vacation in France.
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Before they head to France without him, Kevin's siblings and cousins are constantly teasing and mocking him.
While he's going around trying to figure out what to pack in his suitcase, one of his sisters tells him that he's "what the French call les incompétents."
Linnie was probably just brushing up on her basic French before their trip to Paris, but since "les incompétents" just translates to "the incompetent ones" it's not as impressive as she makes it seem — even if it does confuse Kevin.