/Set-UILang: Sets the default system user interface (UI) language that is used in the mounted offline image. If the language ...

/Set-UILang:

  Sets the default system user interface (UI) language that is used in the
  mounted offline image. If the language is not installed in the Windows 
  image, the command will fail.
  This command is not supported against an online image.

    Example:
      DISM.exe /Image:C:	est\offline /Set-UILang:en-US