The phrase "to master the language" means to become extremely proficient and knowledgeable in speaking, understanding, reading, and writing a particular language. Full definition
He or she is less enthusiastic about having to master the language of another subject, especially one that seems irrelevant. (religion-online.org)
They may not master the language milestones at the same time as other children, and it may be a sign of a language or speech delay or disorder. (cdc.gov)
I'm not very good at talking, but I'll pour someone who will help me to master the language perfectly. (loveawake.com)