Advanced Search is multi-language aware and closely follows Vendure's core DefaultSearchPlugin: a storefront can request any language a channel has enabled and still get results, even for products that haven't been translated into that language yet.
The plugin creates one Typesense document per product variant, per channel, per language. For each channel it indexes that channel's availableLanguageCodes (plus its defaultLanguageCode), together with any language the product is actually translated into — not only the languages that happen to have a translation.
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