The config importer changes the source source used by the config installer so that configuration is read from the staging directory during module install during a configuration import. This is done by setting the storage on the config installer. The module installer then gets this and sets it again after every module enable because enabling modules rebuilds the container. Since the module installer is persisted through module installs the config importer sure set something on the module installer to use the alternative config storage and then we can simplify the config installer.
This is a bug because the config installer is re-entrant. If a language is created during an install then during the config save event the config installer is called to create any language config overrides.