js: add Indonesian translation (id)

[suhindra]

Checklist

• All new and existing tests are passing
• (If adding features:) I have added tests to cover my changes
• (If docs changes needed:) I have updated the documentation accordingly.
• I have added an entry to CHANGES.rst because this is a user-facing change or an important bugfix
• I have written proper commit message(s)

What changes does this Pull Request introduce?

This PR adds Indonesian (id) translation for Isso’s client UI.

Included changes

• Added new translation file: isso/js/app/i18n/id.js
• Registered id in the i18n catalogue: isso/js/app/i18n.js
• Added id to pluralization handling (2-form plural rule)
• Added changelog entry in CHANGES.rst

Translation notes

• Uses the same key set as en.js
• Preserves plural separator format (\n) used by Isso i18n
• Uses concise, UI-friendly Indonesian wording

Why is this necessary?

Isso supports multiple languages and currently has no Indonesian locale.
Adding id improves usability for Indonesian-speaking users by showing native UI strings instead of fallback English.

Testing

npm run test-unit -- i18n-overrides.test.js

[suhindra]

Thanks!