There exist many different languages of Windows Server ISO files. When a user tries to install using the stock Windows task and a non-en-US ISO file, an error shows up in the setupact.log file about the UILanguage property since the unattended.xml file currently depends on that language being present.
A better approach would be to add a standardized metadata value, e.g. win_language, that will be used in the unattended.xml.erb. If a user has an ISO with a different default language, the user can place the language string in a policy's node_metadata so it will be applied to all bound nodes.
Additionally, there are several other language-related properties which default to "en-US". These values should follow the same pattern.
New step in workflow using Japan as example (from previous Slack bug report):
When creating the Windows policy, use node_metadata that includes "win_language" with value "ja-JP".