If you want to completely change Office to a different language, you'll get the best results if you first set that to be your default display language.
For more information on doing that see Change the language Office uses in its menus and proofing tools.
To change the editing language, go to Tools > Language, choose a language, and select OK.
If you want to completely change Office, permanently, to a different language you'll get the best results if you first set that to be your default display language in Windows.
For more information on doing that see Manage display language settings in Windows.
