Five-year-old Tahani is entering a school where English is the only language spoken, but in her home, her family uses a combination of both English and her family's native language. When she starts school, she takes a test measuring her English language development. Her score is well below that of the other children in her class, most of whom live in homes where only one language is spoken. Which of the following is the most likely explanation for Tahani's test scores?