Learning a new language is like learning other skills. You must practice it daily in order to learn it and maintain it. Children can easily learn more than one language. They will likely have a dominant language but even that may change over time. Practice makes perfect.
Language acquisition falls into two categories when learning the second language: first language acquisition and second language acquisition. Everyone learns a first language. Second language acquisition then assumes knowledge in a first language and this influences the learning of the second language in terms of vocabulary, grammatical structures, and writing systems.
There are six stages to language acquisition. They are: pre-production, early production, speech emergent, beginning fluency, intermediate fluency, and advanced fluency. To encourage language acquisition, you can read to your child so they may hear the target language, listen to music, check on their comprehension of the language, have them speak, write, and read in the target language. Language acquisition is a long process that may take years to accomplish but it is also very rewarding.