fix: Handle overlapping custom locale prefixes correctly (#1343)

Consider this config:

```tsx
export const routing = defineRouting({
  locales: ["en-US", "es-US"],
  defaultLocale: "en-US",
  localePrefix: {
    mode: "always",
    prefixes: {
      "es-US": "/us/es",
      "en-US": "/us"
    },
  }
});
```

… a pathname like `/us/es` would previously be incorrectly rewritten to
`/en-US/es` internally. With this change, we now prioritize more
specific prefixes in order to rewrite this request correctly to
`/es-US`.

Fixes #1329

packages/next-intl/src/middleware/utils.test.tsx

@@ -3,6 +3,7 @@ import {
   formatPathnameTemplate,
   getInternalTemplate,
   getNormalizedPathname,
+  getPathnameMatch,
   getRouteParams
 } from './utils';
 
@@ -168,3 +169,36 @@ describe('getInternalTemplate', () => {
     ]);
   });
 });
+
+describe('getPathnameMatch', () => {
+  it('prioritizes more specific custom prefixes for overlapping ones', () => {
+    const locales = ['de', 'de-at', 'de-at-x-test'] as const;
+    const localePrefix = {
+      mode: 'always',
+      prefixes: {
+        de: '/de',
+        'de-at': '/de/at',
+        // Longer locale, shorter prefix
+        'de-at-x-test': '/de/a'
+      }
+    } as const;
+
+    expect(getPathnameMatch('/de/at/test', locales, localePrefix)).toEqual({
+      locale: 'de-at',
+      prefix: '/de/at',
+      exact: true,
+      matchedPrefix: '/de/at'
+    });
+  });
+
+  it('does not confuse unrelated parts of the pathname with a locale', () => {
+    expect(
+      getPathnameMatch('/de/ats', ['de', 'de-at'], {mode: 'always'})
+    ).toEqual({
+      locale: 'de',
+      prefix: '/de',
+      exact: true,
+      matchedPrefix: '/de'
+    });
+  });
+});

packages/next-intl/src/middleware/utils.tsx

@@ -151,6 +151,9 @@ export function getPathnameMatch<AppLocales extends Locales>(
   | undefined {
   const localePrefixes = getLocalePrefixes(locales, localePrefix);
 
+  // More specific ones first
+  localePrefixes.sort((a, b) => b[1].length - a[1].length);
+
   for (const [locale, prefix] of localePrefixes) {
     let exact, matches;
     if (pathname === prefix || pathname.startsWith(prefix + '/')) {