fix: Add server side loading of fallbackLng (#970)

src/config/createConfig.test.ts

@@ -69,6 +69,34 @@ describe('createConfig', () => {
         } as UserConfig)
         expect((config.detection as any).hello).toEqual('world')
       })
+
+      describe('fallbackLng', () => {
+        it('automatically sets if it user does not provide', () => {
+          const config = createConfig({
+            lng: 'en',
+          } as UserConfig)
+
+          expect(config.fallbackLng).toBe('en')
+        })
+
+        it('does not overwrite user provided value', () => {
+          const config = createConfig({
+            fallbackLng: 'hello-world',
+            lng: 'en',
+          } as UserConfig)
+
+          expect(config.fallbackLng).toBe('hello-world')
+        })
+
+        it('does not overwrite user provided boolean', () => {
+          const config = createConfig({
+            fallbackLng: false,
+            lng: 'en',
+          } as UserConfig)
+
+          expect(config.fallbackLng).toBe(false)
+        })
+      })
     })
 
     describe('when filesystem is missing defaultNS', () => {

src/config/createConfig.ts

@@ -37,6 +37,10 @@ export const createConfig = (userConfig: UserConfig): InternalConfig => {
     return combinedConfig as InternalConfig
   }
 
+  if (typeof combinedConfig.fallbackLng === 'undefined') {
+    combinedConfig.fallbackLng = combinedConfig.defaultLocale
+  }
+
   if (!process.browser) {
     combinedConfig.preload = locales
 

src/serverSideTranslations.ts

@@ -40,10 +40,16 @@ export const serverSideTranslations = async (
     [initialLocale]: {},
   }
 
+  if (typeof config.fallbackLng === 'string') {
+    initialI18nStore[config.fallbackLng] = {}
+  }
+
   namespacesRequired.forEach((ns) => {
-    initialI18nStore[initialLocale][ns] = (
-      (i18n.services.resourceStore.data[initialLocale] || {})[ns] || {}
-    )
+    for (const locale in initialI18nStore) {
+      initialI18nStore[locale][ns] = (
+        (i18n.services.resourceStore.data[locale] || {})[ns] || {}
+      )
+    }
   })
 
   return {