The "subsites and second languages feature" is developed only for administrators, they trust each other, or for only one (1) administrator for all subsites and second languages.
For experts there are some options to compromise other subsites or the main website, if they have access to the backend of one of them.
Second languages and subsites are working on the same principle, they are independent sites that use the same CMSimple installation.
The only difference is, that second languages automatically appears in the language menu.
Both, the main site and subsites, can be multilingual, and are together with the respective second languages, a group with a main language (language_default), which is defined in the relevant main page or subsite by configuration.
Second languages recognize their language itself, by the 2-characters folder (de, en, fr ...), in which they are running.