/Set-SysLocale: Sets the language for non-Unicode programs (also called system locale) and font settings in the mounted offline ...

/Set-SysLocale:

  Sets the language for non-Unicode programs (also called system locale) and
  font settings in the mounted offline image.
  This command is not supported against an online image.

  WARNING: You cannot set Unicode-only languages as the system locale. If you
           try, the /Set-SysLocale option will fail and the language for 
           non-Unicode programs will not be changed.

    Example:
      DISM.exe /Image:C:\test\offline /Set-SysLocale:en-US