Comparing DTD-s for component (component-install.dtd) and plugin (plugin-install.dtd) install, I found, that:

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!!!)

Moving to Q&T for feedback.

Q&T Note; marking as under review