The languages/ directory can be added to project_copy_folders for having them copied in place. And they belong to this than rather to project_shared_children option because they are tied to a particular deploy (core version, plugins).
But what if the languages/ folder already contains files from the deploy, e.g. for overriding or adding (yet missing) plugin or theme translations? Then the folder won’t be copied and the one from the deploy is used, which doesn’t solve the issue.