Q. How do I inject a language pack into an offline-mounted WIM image?

A. Once you have a WIM file mounted to a folder, as in

DISM.exe /mount-wim /wimfile:<WIM name and location, c:\wims\install.wim> /index:<image within WIM, e.g. 1> /mountdir:<mount location, e.g. c:\mount

You can inject drivers using the command:

dism /image:<WIM mounted image location> /scratchdir:<scratch location> /add-package /packagepath:<location of language cab and cab name, e.g. c:\languages\lp_en-us.cab>

Once you've made the changes to your WIM, make sure you commit the changes. When you unmount, use the command

dism /commit-wim /mountdir:<mounted location>