Thank you for reporting a bug to Unity.
After reviewing the issue, we've confirmed it aligns with the current design and intended localization. In particular, a Shadowmask (written as one word, intended as a proper noun) is a specific Unity concept within the Editor, and is localized as such, by design.
We will close this case as 'As Designed.' If you have feedback, please let us know - we value your input and consider it in future improvements.