Mhhh...

I don't understand why we would have to 'add' an other language.

If someone wants to generate the doxydoc, he has to install doxygen,
graphviz and so on.
If he also wants to generate the doc of lua scripts (such as
script_helpers.lua), then he will also have to install lua2dox.

What we just have to do is to change our doxyfile to instruct doxygen
to filter lua files through that lua2dox. And if the user did not
install it, doxygen will just complain about a not found file, and do its work
anyhow.