In Joomla, the file index.html is no longer used in the directories.

However, in the file install.xml the following line of code is there.
index.html

The file install.xml is available in both the directories \administrator\language\en-GB and \language\en-GB.

Since en-GB is native to Joomla and not getting installed just like any other language pack, the issue is not seen. If someone copies thse file to create a language pack, the language pack installation will fail as there would not be any file named index.html.

Action to be taken:
Please remove the above mentioned tag from the install.xml files that are available in the directories mentioned.