In CMSimple 4.0 or higher you can make the contents of the site invisible for visitors. The contents are only visible, if you are logged in as administrator. For the visitor a message will appear. This message can be changed in the language file, so that it can be adapted to the needs.
The administrator gets a permanent message in the backend, that the site is in maintenance mode. So he should not forget to disable the maintenance mode before logging out.
The maintenance mode can be activated in the CMS configuration, so you can make the contents of the main installation, second languages and subsites independently visible or invisible:
Settings => CMS => Site => Content_visible:
CMSimple 4.0 or higher has its own maintenance template, that can be customized easily.