> Most router admin panels are probably written in C++
What makes you say that? The modern routers I've used all run a full embedded Linux like BusyBox, and they have no need for compiled high performance panels. It'd be more expensive and less portable to write the admin panel in C++, and I can't really think of any strong benefits that would make it worth using C++. I would have assumed they run a scripting language. But I'm checking some of the OS projects now instead of assuming.
Poking around, I see that Tomato is a busybox distribution, all the Linux tools including httpd are C (not C++), and the admin panal pages are .asp (written in VBScript).
DD-wrt appears to have PHP admin panels.
Looks like Asuswrt-Merlin uses .asp.
M0n0wall is all php.
Let me know if you find one with admin panels in C++, I'm interested.