fix: detect language for _next/data requests (#21696)

middleware/detect-language.js

@@ -35,12 +35,14 @@ function getUserLanguage(browserLanguages) {
 }
 
 export default function detectLanguage(req, res, next) {
-  // determine language code from first part of URL, or default to English
+  // determine language code from the URL, or default to English
   // /en/articles/foo
   //  ^^
-  const firstPartOfPath = req.path.split('/')[1]
+  // /_next/data/development/en/articles/foo
+  //                         ^^
+  const maybeLanguage = req.path.split('/')[req.path.startsWith('/_next/data/') ? 4 : 1]
 
-  req.language = languageCodes.includes(firstPartOfPath) ? firstPartOfPath : 'en'
+  req.language = languageCodes.includes(maybeLanguage) ? maybeLanguage : 'en'
   // Detecting browser language by user preference
   const browserLanguages = parser.parse(req.headers['accept-language'])
   req.userLanguage = getUserLanguage(browserLanguages)