/Set-UILangFallback: Sets the fallback default language for the system UI in the mounted offline image. This setting is used ...

/Set-UILangFallback:

  Sets the fallback default language for the system UI in the mounted offline
  image. This setting is used only when the language specified by the 
  /Set-UILang option is a partially localized language.
  This command is not supported against an online image.

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