If the Office Home and Student 2013 RT language changed to English after you restarted Windows RT, follow these steps to change the language back:
From the Windows Start screen, type compmgmt.msc.
Tap or click compmgmt.msc on the left to open the Computer Management window.
Under System Tools on the left, browse to the MUI folder (open Task Scheduler > Microsoft > Windows > MUI).
Tap and hold or right-click LPRemove and choose Run from the list.
Wait a few minutes, and then start Office Home and Student 2013 RT. It should now be in the original language.
Note: You'll need to be in an administrator account for these steps to work. See How do I know I'm signed in as an administrator? if you're not sure which type of account you have.
Choosing Restart from the Settings charm in Windows RT can cause this problem if the device is trying to install the update to upgrade from the preview to the final edition of Office.