MOBILE-4288 multilang: Fix unknown language filter

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

@@ -47,10 +47,12 @@ export class AddonFilterMultilangHandlerService extends CoreFilterDefaultHandler
         // Get available languages.
         const regex = /<(?:lang|span)[^>]+lang="([a-zA-Z0-9_-]+)"[^>]*>.*?<\/(?:lang|span)>/img;
         const languages: Set<string> = new Set();
+        let firstLanguage: string | undefined;
         let match: RegExpExecArray | null;
 
         while ((match = regex.exec(text))) {
             const language = match[1].toLowerCase().replace(/_/g, '-');
+            firstLanguage = firstLanguage ?? language;
 
             languages.add(language);
         }
@@ -62,6 +64,10 @@ export class AddonFilterMultilangHandlerService extends CoreFilterDefaultHandler
             language = CoreLang.getParentLanguage();
         }
 
+        if (!language) {
+            language = firstLanguage;
+        }
+
         if (!language) {
             return text;
         }

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

@@ -122,6 +122,25 @@ describe('Lang', () => {
         `, 'Japanese text');
     });
 
+    it('filters multilang text using unknown language', async () => {
+        currentLanguage = 'ca';
+        parentLanguage = undefined;
+
+        await expectMultilangFilter(`
+            <span class="multilang" lang="es">Spanish</span>
+            <span class="multilang" lang="en">English</span>
+            <span class="multilang" lang="ja">Japanese</span>
+            text
+        `, 'Spanish text');
+
+        await expectMultilangFilter(`
+            {mlang es}Spanish{mlang}
+            {mlang en}English{mlang}
+            {mlang ja}Japanese{mlang}
+            text
+        `, 'text');
+    });
+
     /**
      * Test multilang filter (normalizing whitespace).
      */