If a subsite or second language is operated by its own author, it may be useful to provide own language files for this subsite or second language.
This has the nice effect, that the author of this subsite or second language does not have a backend access to the language files of the main site, and thus he can not cause damage there.
To provide own language files is simple:
Create a folder "languages" directly in the root of each subsite or second language and give it writing permissions.
Copy the language files from the folder:
from the main installation to the folder "languages" of the subsite and give them writing permissions. This subsite is now working with their own language files.