Why use _translations query if language filter already returns all translations?

The _translations query serves a different purpose than your base query with language == $language. Here's why you need it:

Your initial query *[_type == "lesson" && language == $language] only fetches documents in one specific language. The _translations part is what gives you access to all the other language versions of that same content.

Think of it this way:

• The base query finds all lessons in, say, English ($language = "en")
• The _translations subquery then looks up and includes the Spanish, French, German, etc. versions of each lesson

The Document Internationalization plugin creates separate documents for each language version, linked together through a special translation.metadata document. The query pattern you're seeing:

"_translations": *[_type == "translation.metadata" && references(^._id)].translations[].value->

Does three things:

1. Finds the metadata document that references the current lesson (references(^._id))
2. Gets all translation references from that metadata document (.translations[])
3. Resolves those references to get the actual translated documents (.value->)

Why is this useful? It lets you build language switchers, show "Available in: EN, ES, FR" indicators, or prefetch alternate language versions for better UX. Without the _translations query, you'd only have the English lessons with no way to know which other languages exist or how to link to them.

So you're not replacing language == $language with _translations – you're using both together. The first filters to your current language, the second connects you to all the alternatives.