sed script to convert language files back to 1.2 format

Execute with sed -r -f convert.sed -i lang/strings_*.txt
Convert language files back to 1.2 format

This reverts cad0ad10f4101da0c2947acfb75a1fc69b471cac
Removing conversion script
Renaming language readme file
Update phpdoc for $g_custom_strings_file
Documentation: custom strings file

The Custom Strings file is now defined in $g_custom_strings_file.

Create a new section under Customization to describe it, and refer to
the new section from various places in the documentation.
Documentation: various formatting and layout updates
Fix #14099: reverting to old language strings format