How to filter internationalized document references by language in Sanity?

I can help you with filtering references based on the language from @sanity/document-internationalization! The issue in your code is a few syntax errors and you need to use the correct field name that the plugin creates.

When you use @sanity/document-internationalization, it adds a special field called __i18n_lang to your documents to track their language. Here's the corrected filter:

defineField({
  name: 'product',
  type: 'array',
  of: [{type: 'reference', to: [{type: 'dogTreat'}, {type: 'catTreat'}]}],
  options: {
    filter: ({ document }) => {
      const currentLanguage = document.__i18n_lang; // Use __i18n_lang field
      return {
        filter: '_type == $type && __i18n_lang == $language', // Fixed syntax
        params: {
          type: 'catTreat',
          language: currentLanguage
        }
      };
    }
  },
  group: 'products',
})

Key changes I made:

1. Use __i18n_lang field: The document-internationalization plugin stores the language code in a field called __i18n_lang, not language
2. Fixed GROQ syntax: Your filter string had template literals mixed with GROQ syntax incorrectly - use parameterized queries instead
3. Use $language parameter: Pass the language as a param to avoid injection issues

If you want to filter for multiple types, adjust it like this:

filter: ({ document }) => {
  const currentLanguage = document.__i18n_lang;
  return {
    filter: '_type in $types && __i18n_lang == $language',
    params: {
      types: ['catTreat', 'dogTreat'],
      language: currentLanguage
    }
  };
}

This will ensure that when you're editing the en-US version of a document, you'll only see en-US versions of the referenced products in the dropdown. The __i18n_lang field is automatically managed by the plugin alongside __i18n_base and __i18n_refs fields that maintain relationships between translated documents.