Add miscellaneous UI elements for translation

[karwosts]

Proposed change

Add a few strings for localization. Requested from #14416.

Translation keys for:

• The word "unknown" in the devices table.
• The column headers in the developer tools statistics table
• The message "The edit UI is not available when in YAML mode." when editing a yaml-sourced dashboard.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue or discussion: Missing translations #14416
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io