What are the nationalities of the magic weapons? It's obvious that the magic weapons that Nanoha and the rest wield have different nationalities, as Rising Heart speaks English, but Hayate's Knight's weapons don't.

I am wondering: what are the nationalities of the weapons based on?
There is no official answer to the reason for speaking a different language. But, the language is tied into the magic system that the device uses. 

There are: 


English for Mid-Childa devices 
German for the Ancient-Belkan devices 
Japanese for Unison devices


Unison devices are the devices that merge with their master to increase the combat capabilities, such as Reinforce and Agito. The devices that are up for the debate are the Modern-Belkan devices, such as Strada (Erio) using German, while Mach Caliber (Subaru) uses English. 

My personal opinion would be that Modern-Belkan devices capable of transforming form would use German (Strada), while others do not (Mach Caliber).