1. "folder" attribute of "files" element for plugin (but not for component!) has definition, that doesn't allow slashes, so:
- is illegal, but it works OK (as for component, where it is legal).
2. Description of "languages" element is strange:
there is NO way to describe differently language file for SITE and for ADMINISTRATOR (like for component). Currently language files are copied to administrator (backend) only

3. And also I want to mention a BUG installing language files (it's both for plugins and components:
only first "languages" element is being processed, although DTD allows multiples languages elements (the same for installing components), so in my case, if I type this:
en-GB.plg_content_yvcomment.ini
ru-RU.plg_content_yvcomment.ini
- only first language file is installed.
By the way, in my Extension (yvComment) I'm using
this instead:
www/administrator/language/en-GB/en-GB.plg_content_yvcomment.ini
www/administrator/language/ru-RU/ru-RU.plg_content_yvcomment.ini
- but this is not consistent with "folder" attribute usage for files (that works!!!)