Glob its more easier for the user to add new translation files o new modules, make language unnattended. but you are right the fn: GetAvailableSkins() its more faster than glob.
About the cookie yes, using the cookie for the no authenticated user and for registered pilots when they loads settings from their profile db and the next reload use the cookie for use less db querys.
Using the XML, yes, best using other php file like: config.lang.php for admin panel see the autor, license etc etc...
I am testing this, and i will try to integrate the function that you have said instead of Glob().
Tomorrow i send you...
if you need a help with other questions i can help you, or colaborate with the project.
Greetings