✨ Add opt-in client-side language redirect

[aplgr]

Summary

This PR adds an optional client-side language redirect feature for multilingual Blowfish sites.

Related discussion: #2934

The feature is fully opt-in and disabled by default. Existing sites keep their current behavior unless they explicitly enable the new languageRedirect configuration.

Problem

Blowfish already provides a language dropdown for multilingual sites, but visitors still land on the configured default content language first.

For static multilingual sites without server-side language negotiation, this means that a visitor whose browser prefers another supported language has to switch manually, even when a translated version already exists.

Solution

This PR introduces a small client-side redirect helper that:

• detects the browser language on first visit
• redirects only when a matching translation exists
• stores explicit language selections from the existing language dropdown in localStorage
• respects the stored language preference on later visits
• avoids redirect loops
• does not require cookies or any server-side component

The browser-language redirect is limited to home pages by default to avoid surprising users who open deep links. Stored language preferences can still be respected on translated pages when a matching translation exists.

The matching logic preserves locale subtags where available, so sites using variants such as pt-BR / pt-PT or zh-Hans / zh-Hant can still honor the visitor’s exact browser preference before falling back to a base language match.

Configuration

The feature can be enabled with:

[languageRedirect]
  enabled = true
  storageKey = "blowfish_preferred_language"
  fallbackLanguage = "en"
  browserRedirectHomeOnly = true
  storedLanguageRedirect = true

Backward compatibility

This is disabled by default.

No existing multilingual behavior changes unless the site owner explicitly enables:

[languageRedirect]
  enabled = true

Notes

The implementation is intentionally client-side only and uses the existing Hugo/Blowfish multilingual data, especially .AllTranslations, instead of introducing any server-side requirements or external dependencies.

[netlify]

✅ Deploy Preview for snazzy-dango-efb2ec ready!

Name	Link
🔨 Latest commit	6b11b81
🔍 Latest deploy log	https://app.netlify.com/projects/snazzy-dango-efb2ec/deploys/6a05cce98fe2c70008110421
😎 Deploy Preview	https://deploy-preview-2935--snazzy-dango-efb2ec.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 3a86a7656e

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".