>They're comparing them to the performance of a person who is a native speaker of both of the languages in question.
Which is synonymous with the best translators in the world. Those people are relatively few and far between honestly - I've traveled a lot and I'd argue that native bi-lingual people are quite rare.
Depends on which part of the world you're in. Have you been to the USA? English/Spanish bilingualism is pretty common there. And there are lots of places where it's completely unremarkable for children to grow up speaking two languages.
Which is synonymous with the best translators in the world. Those people are relatively few and far between honestly - I've traveled a lot and I'd argue that native bi-lingual people are quite rare.