Final answer:
The statement that is not true is that "Scientists understand how infants learn language." Despite infants' abilities to distinguish languages and even learn sign language, the complex process of language acquisition remains not fully understood.
Step-by-step explanation:
You asked which of these statements about language is not true:
- Infants are capable of differentiating more than one language.
- Scientists understand how infants learn language.
- First words are understood around eight to nine months.
- Infants can learn sign language.
The statement that is not true is "Scientists understand how infants learn language". While infants demonstrate an exceptional ability to distinguish between different languages and can even learn sign language, scientists still do not fully understand the complexities of how language acquisition occurs. Infants can indeed differentiate more than one language from birth and can understand first words around eight to nine months. Moreover, they are capable of learning sign language.