fix: add date picker placeholder for Icelandic

[ofurtumi]

Closes #9599

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

Testing a date picker with the locale set to "is-IS" should produce the placeholder dd.mm.áááá

🧢 Your Project: