AI: address issue #436

frontend/src/entry-server.tsx

@@ -1,6 +1,6 @@
 import { QueryClient, QueryClientProvider, dehydrate, skipToken } from '@tanstack/react-query'
 import { PassThrough } from 'node:stream'
-import { renderToPipeableStream } from 'react-dom/server'
+import { renderToPipeableStream, renderToStaticMarkup } from 'react-dom/server'
 import { HelmetProvider } from 'react-helmet-async'
 import { I18nextProvider } from 'react-i18next'
 import { createStaticHandler, createStaticRouter,
@@ -9,7 +9,7 @@ import { createStaticHandler, createStaticRouter,
 import { AppFrame, appRoutes, resolvePageRoute } from '@/App'
 import { apiGet, isApiError } from '@/lib/api'
 import { createUII18n } from '@/lib/i18n'
-import { fetchLocales, isLocaleCode, legacyLocalePath } from '@/lib/locales'
+import { fetchLocales, isLocaleCode, legacyLocalePath, switchLocalePath } from '@/lib/locales'
 import { localesKeys, sessionKeys } from '@/lib/queryKeys'
 import { withRenderContext } from '@/lib/renderContext.server'
 
@@ -171,8 +171,26 @@ export const render = async (
 	}))
       }
       const helmet = helmetContext.helmet!
+      const alternates = status === 200 && known != null
+	  ? await Promise.all (locales.locales.map (entry => entry.code)
+	      .filter (isLocaleCode)
+	      .filter (code => known.route.handle?.locale == null
+		  || known.route.handle.locale === code)
+	      .map (async targetLocale => {
+		const location = { pathname: url.pathname, search: url.search, hash: '' }
+		const path = targetLocale === locale
+		    ? url.pathname + url.search
+		    : (known.route.handle?.switchLocale != null
+		       ? await known.route.handle.switchLocale ({
+			   location, params: known.params, locale, targetLocale })
+		       : switchLocalePath (location, targetLocale))
+		return <link key={targetLocale} rel="alternate" hrefLang={targetLocale}
+		    href={new URL (path, origin).href}/>
+	      }))
+	  : []
       const head = [helmet.title, helmet.meta, helmet.link, helmet.style]
 	  .map (entry => entry.toString ()).join ('')
+	  + renderToStaticMarkup (<>{alternates}</>)
       const dehydrated = dehydrate (client, { shouldDehydrateQuery: query =>
 	  query.state.status !== 'pending'
 	  && query.meta?.clientOnly !== true