Good question, but how hard is that to change voices ?
The default voices should be english yes (though Saruman is great in german), but german could still be used for german players.. ?
And why not a french version... ! If the handling is not so hard and once the game reaches a stable version, of course. And if we have french voices.
For my part Edain mod is installed on a french version of the game and it works fine. Some voices are still in french and it's nice (Gondor archers..). I lost Boromir, who is in english now (a shame, he had a great voice in french ^^).
Just for joke, here is a video I made with the old Edain Mod (it was to create a mod for skyrim that looks like Saruman's blast/fireball):
https://www.youtube.com/watch?v=X1mQK_7Vfk8"A moi, mes créatures !!" is french ! The other sentences are in german (and it's fine).