/Set-UserLocale: Sets the "standards and formats" language (also called user locale) in the mounted offline image. The "Standards ...

/Set-UserLocale:

  Sets the "standards and formats" language (also called user locale) in the
  mounted offline image. The "Standards and formats" language is a
  per-user setting that determines default sort order and the default settings
  for formatting dates, times, currency, and numbers.
  This command is not supported against an online image.

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