Complete remaining SEO optimization for Japanese docs site

Goal

Complete remaining SEO optimization work for the Japanese docs site.

Current status

Core SEO elements (hreflang tags and canonical URLs) were implemented and deployed in July 2025:

1. hreflang tags implemented in issue Implement hreflang Tags for Docs Site Localization (#313 - closed) , MR Add hreflang logic and consolidate canonical pa... (gitlab-org/technical-writing/docs-gitlab-com!949 - merged)
2. Canonical URLs with English fallback implemented in Configure English fallback for untranslated Jap... (gitlab-org/technical-writing/docs-gitlab-com!833 - merged)

The above implementation exists but is not functioning correctly when Japanese is enabled

Engineering goal of this issue

Debug and fix why existing implementation doesn't work when Japanese is enabled, and address the blockers mentioned in detail in #280 (comment 2887663220)

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The following items will be tracked in a separate issue Done ✔️

Additional SEO work

• Validate current implementation , i.e. audit hreflang tags and canonical URLs
• Implement language specific sitemaps , e.g. sitemap-en.xml and sitemap-ja.xml, submit to Google Search Console
• Verify meta descriptions display in Japanese