I see your point, and I always make a point of deleting manually all unnecessary language files from any program.
The trouble is, Piriform software has options within it's software to change the language after install, and in multi computer environments this option may be used. Removing language files during installation would remove optional languages from the drop down language setting menu, making it's presence kinda useless.
That's my take on it, and I can't really see this change being made by Piriform.