i18n fallback does not work with manual routing

[Alejandro-FA]

Astro Info

Astro                    v4.16.12
Node                     v22.11.0
System                   macOS (arm64)
Package Manager          npm
Output                   static
Adapter                  none
Integrations             @astrojs/sitemap

If this issue only occurs in one browser, which browser is a problem?

No response

Describe the Bug

The i18n.fallback and i18n.routing.fallbackType options are ignored by the middleware function from astro:i18n when using manual routing.

As far as I understand, the following two setups should have the same behavior:

Configuration 1 (default routing middleware)

Configure astro.config.mjs to set the fallback of the es locale to en. Furthermore, specify the fallbackType to "redirect".

import { defineConfig } from "astro/config"
export default defineConfig({
  i18n: {
    defaultLocale: "en",
    locales: ["en", "es"],
    fallback: { es: "en" },
    routing: {
      prefixDefaultLocale: true,
      fallbackType: "redirect",
    },
  }
})

Configuration 2 (manual routing)

Configure astro.config.mjs to set the fallback of the es locale to en.

import { defineConfig } from "astro/config"
export default defineConfig({
  i18n: {
    defaultLocale: "en",
    locales: ["en", "es"],
    fallback: { es: "en" },
    routing: "manual",
  }
})

Use the middleware function in src/middleware.ts to recover the default routing behavior (and set the fallbackType to `"redirect" as done before)

import { sequence } from "astro:middleware";
import { middleware } from "astro:i18n";

export const onRequest = sequence(
  middleware({
    prefixDefaultLocale: true,
    redirectToDefaultLocale: true,
    fallbackType: "redirect",
  }),
);

Behavior

With the first configuration, navigating to /es correctly redirects to /en if src/pages/es/index.astro does not exist. However, with the second configuration, navigating to /es throws a 404 error instead of redirecting to /en.

What's the expected result?

The fallback and fallbackType options behave in manual routing exactly as in "automatic" routing.

Link to Minimal Reproducible Example

https://stackblitz.com/edit/github-wtyvk1?file=src%2Fmiddleware.ts

Participation

• I am willing to submit a pull request for this issue.