i18n URL strategy for an Astro site

Decision

• Use a subdirectory prefix URL structure.
• Set prefixDefaultLocale: false in Astro i18n.
• Default language (ja) stays at the root (e.g. /posts/slug).
• Additional languages (en, etc.) use prefixes (e.g. /en/posts/slug).
• Keep the same slug across languages.

Context

We needed to support multilingual content using AI/translation workflows while preserving the following constraints:

• Keep existing Japanese URLs intact to preserve SEO and inbound links.
• Stay aligned with Astro’s file-based routing for low maintenance.
• Treat Japanese as the source of truth.

Options

• Option A (Suffix): /posts/slug/en
  • Poor fit with Astro’s standard i18n routing.
• Option B (Root English): / as English and move Japanese to /ja/
  • Requires changing existing Japanese URLs.
• Option C (Prefix + JA Default): Keep / as Japanese and put English under /en/
  • Preserves URLs while enabling i18n.

Rationale

• Backward compatibility: No URL changes for existing Japanese content.
• Lower implementation cost: Achievable with Astro’s built-in i18n configuration.
• Quality and trust: Easier to separate original content from translations.

Consequences

• Non-Japanese visitors landing on / will see Japanese.
• If needed, add a language suggestion UI or an Accept-Language based redirect.
• hreflang and cross-language links are easy to generate with Astro helpers.
• Enforcing identical slugs across languages becomes a content management rule.