If you want to replace the files and this maybe risky...
in the Awizard thing once installed in your pc you'll find a in the "BIN" folder a CAB file named unlock or something of sorts.
copy this file to the device, run it, after it with Resco File explorer (follow Erezyad method) you'll see a completely new folder named "2" there is a replica of the root folder of your device...
i have tried adding files to one or the other and while i cannot modify some files on the root (inside windows folder i.example) i can write or erase anything i want form the contents of the "2" folder and even better it doesnt mirrors the root neithe rthe root mirrors the 2...
so my guess is that on hard reset the contents of 2 are copied to \root and.
i'm too lazy to hard reset my device and i ma happy with my french language .. however you could try for example.
from folder 2 erase all the sample pictures that comes with the OS... hard reset and see if you get them... if you dont i am right and you can try and write the english .mui files in the folder 2 and hard reset... or you can end up briking your device... but it is worth a try isnt it? you look stuck anyway
let me know
PS: You can easily copy the contents of 2 folder to your pc before messing with it (no sharing violation or anything (that could act as a backup )