🌐 refactor: improve last_updated_on format

[welpo]

Summary

Changes the last_updated_on i18n string to (1) include a $DATE placeholder and (2) be less verbose.

The placeholder is useful for languages like Japanese. Before, all languages were forced to the last_updated_on string + $date structure. The new format improves translation accuracy.

Related issue

Idea from #330.

Changes

• Update i18n strings
• Implement $DATE placeholder replacement

Accessibility

Remains the same.

Screenshots

Old

New

Type of change

• Bug fix (fixes an issue without altering functionality)
• New feature (adds non-breaking functionality)
• Breaking change (alters existing functionality)
• UI/UX improvement (enhances user interface without altering functionality)
• Refactor (improves code quality without altering functionality)
• Documentation update
• Other (please describe below)

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Checklist

• I have verified the accessibility of my changes
• I have tested all possible scenarios for this change
• I have updated theme.toml with a sane default for the feature
• I have made corresponding changes to the documentation:
  • Updated config.toml comments
  • Updated theme.toml comments
  • Updated "Mastering tabi" post in English
  • (Optional) Updated "Mastering tabi" post in Spanish
  • (Optional) Updated "Mastering tabi" post in Catalan

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