Add missing translations in greek

[vadal]

We are improving the internationalization of the PyLadiesCon 2025 website, and several strings are still missing from the Greek (el) translation file.

✅ What needs to be done

• Review the Greek translation file for missing translations
• Review the English source strings (hardcoded text not wrapped with t()), starting from CoC, Sponsors, and Schedule pages.
• Add any missing translation keys.
• Ensure the structure matches the original by following the translation guide.

Small PRs are welcome!

Thanks for helping make the PyLadiesCon website more inclusive and accessible! 💜