fix: fallbackLocales overriden if parent found

Author: Sergio

packages/conf/src/index.test.ts

@@ -173,7 +173,7 @@ describe("@lingui/conf", function () {
         ),
       })
       expect(config.fallbackLocales).toEqual({
-        "en-US": "en",
+        "en-US": ["en"],
         default: "en",
         "es-MX": "es"
       })

packages/conf/src/index.ts

@@ -278,7 +278,7 @@ export function fallbackLanguageMigration(
 
   config.locales.forEach((locale) => {
     const fl = getCldrParentLocale(locale.toLowerCase())
-    if (fl) {
+    if (fl && !config.fallbackLocales[locale]) {
       config.fallbackLocales = {
         ...config.fallbackLocales,
         [locale]: fl