MOBILE-4288 multilang: Use fallback language

src/addons/filter/multilang/services/handlers/multilang.ts

@@ -58,15 +58,13 @@ export class AddonFilterMultilangHandlerService extends CoreFilterDefaultHandler
         }
 
         // Find language to use.
-        let language: string | undefined = await CoreLang.getCurrentLanguage();
-
-        if (!languages.has(language)) {
-            language = CoreLang.getParentLanguage();
-        }
-
-        if (!language) {
-            language = firstLanguage;
-        }
+        const language = [
+            await CoreLang.getCurrentLanguage(),
+            CoreLang.getParentLanguage(),
+            CoreLang.getFallbackLanguage(),
+            firstLanguage,
+        ]
+            .find(candidate => candidate && languages.has(candidate));
 
         if (!language) {
             return text;

src/core/services/tests/lang.test.ts

@@ -122,7 +122,7 @@ describe('Lang', () => {
         `, 'Japanese text');
     });
 
-    it('filters multilang text using unknown language', async () => {
+    it('filters multilang text using the fallback language', async () => {
         currentLanguage = 'ca';
         parentLanguage = undefined;
 
@@ -131,7 +131,7 @@ describe('Lang', () => {
             <span class="multilang" lang="en">English</span>
             <span class="multilang" lang="ja">Japanese</span>
             text
-        `, 'Spanish text');
+        `, 'English text');
 
         await expectMultilangFilter(`
             {mlang es}Spanish{mlang}
@@ -141,6 +141,23 @@ describe('Lang', () => {
         `, 'text');
     });
 
+    it('filters multilang text using the first language', async () => {
+        currentLanguage = 'ca';
+        parentLanguage = undefined;
+
+        await expectMultilangFilter(`
+            <span class="multilang" lang="es">Spanish</span>
+            <span class="multilang" lang="ja">Japanese</span>
+            text
+        `, 'Spanish text');
+
+        await expectMultilangFilter(`
+            {mlang es}Spanish{mlang}
+            {mlang ja}Japanese{mlang}
+            text
+        `, 'text');
+    });
+
     /**
      * Test multilang filter (normalizing whitespace).
      */