In my opinion You don't learn language You acquire  it in the same way You acquire your first language .

i try to understand why we lose the language which you learn  if we don't use it for two or 3 weeks i found after 15 days Your brain start to  forget this language  gradually and if we ask Ourself  why we don't lose our mother tongue after this period , we will find that it depend in our brain  specifically depends on neuroplasticity  when you learn  language chemical signals increase changes in your short term memory but induce didn't change the structural  changes in your long term memory .

change long term memory depends on behavior and to change Your behavior you have to practice so if you don't use it lose it