feat: Add EN fallback to blog (parity with docs)

[kohbis]

Desired feature or idea

Docs fall back to EN when a translation is missing, but blog does not.
Adding the same EN fallback for blog would help users, and should increase the chances that blog content gets read.

Acceptance:

• Non‑EN /blog/ renders EN posts if no translation exists
• Localized posts override EN

Additional context

Sample implementation: https://github.com/open-telemetry/opentelemetry.io/pull/8940/changes

[chalin]

Thanks for the issue @kohbis. @kohbis and @open-telemetry/docs-maintainers: I'll explain shortly (as soon as I can) why we probably don't want to do this -- eventhough it is easy to implement.

[chalin]

There might be a fresh Hugo feature that could help us here. Stay tuned.

[chalin]

Tracker / parent issue (FYI):

• [i18n] Setup en-fallback for sections other than docs #6338