There is a pretty famous research paper called "Cross-language speech perception: Evidence for perceptual reorganization during the first year of life" by Werker and Tees that shows that infants are better at picking up subtleties of phonemes and they start focusing on the most heard language at around 9 to 12 months of age.

The paper is pretty old, published in 1984, but it was very influential at that time. Janet Werker has several other studies about language acquisition, so she might be a good name to start checking when you're interested in that topic.