Fix redirect & changeLocale with prefix path

[fujikky]

I found the issue that redirect and changeLocale function are wrong when using gatsby's prefix path (like /intl).

redirect:

• Expected: /intl/ -> /intl/en/
• Actual: /intl/ -> /intl/en/intl/

changeLocale function (en -> de):

• Expected: /intl/en/ -> /intl/de/
• Actual: /intl/en/ -> /intl/de/en/

You can reproduce this problem with gatsby-starter-default-intl using this patch.

diff --git a/examples/gatsby-starter-default-intl/gatsby-config.js b/examples/gatsby-starter-default-intl/gatsby-config.js
index 24c6a80..165d2f7 100644
--- a/examples/gatsby-starter-default-intl/gatsby-config.js
+++ b/examples/gatsby-starter-default-intl/gatsby-config.js
@@ -1,4 +1,5 @@
 module.exports = {
+  pathPrefix: `/intl`,
   siteMetadata: {
     title: `Gatsby Default Starter`,
     description: `Kick off your next, great Gatsby project with this default starter. This barebones starter ships with the main Gatsby configuration files you might need.`,
diff --git a/examples/gatsby-starter-default-intl/package.json b/examples/gatsby-starter-default-intl/package.json
index 403542a..88d9615 100644
--- a/examples/gatsby-starter-default-intl/package.json
+++ b/examples/gatsby-starter-default-intl/package.json
@@ -27,11 +27,11 @@
   ],
   "license": "MIT",
   "scripts": {
-    "build": "gatsby build",
+    "build": "gatsby build --prefix-paths",
     "develop": "gatsby develop",
     "format": "prettier --write src/**/*.{js,jsx}",
     "start": "npm run develop",
-    "serve": "gatsby serve",
+    "serve": "gatsby serve --prefix-paths",
     "test": "echo \"Write tests! -> https://gatsby.dev/unit-testing\""
   },
   "repository": {

and run: $ yarn build && yarn serve && open http://localhost:9000/intl/

[alfonmga]

I'm using redirect: false gatsby-plugin-intl config option and when I use changeLocale function to switch to default language it adds /en URL prefix.

Is this a bug? I don't would like to use a prefix URL path for the default language.
Also I think /en URL path aka default language should throw out an 404 error in this scenario, it would make sense to avoid duplicate content.

This is the repo btw: https://github.com/voicerum/homepage/tree/c76c80a7c1efe5a35ba3043fd338d3aa09feacd5