Its about native language speakers going into the country they claim as residents.
Border and Customs agents are naturally suspicious of persons who cannot speak, or do so with heavy accent, the native language of the nation they purport to be legal resident of.
Border agents are trained to identify persons with native speech, those with trained speech.
When I lived in Canada as a legal resident , working for American companies, I had occasion to cross the border many times. On both sides I was gven easy passage due to ID and language. Kansas warsher instead of washer, ruff instead of roof, Oklahoma ' you know it' Manitoba had many others peculiar to them .
Peculiarities in speech and behaviour are the study of our border agents whether they question in depth or pass you by. Even persons affluent in multiple languages have signature accents, formation of language even in sign that can alert a trained agent of impropriety