diff --git a/src/providers/lang.ts b/src/providers/lang.ts
index 621a89a19..266421412 100644
--- a/src/providers/lang.ts
+++ b/src/providers/lang.ts
@@ -58,6 +58,8 @@ export class CoreLangProvider {
      * @param {string} [prefix] A prefix to add to all keys.
      */
     addSitePluginsStrings(lang: string, strings: any, prefix?: string): void {
+        lang = lang.replace('_', '-'); // Use the app format instead of Moodle format.
+
         // Initialize structure if it doesn't exist.
         if (!this.sitePluginsStrings[lang]) {
             this.sitePluginsStrings[lang] = {};
@@ -305,7 +307,7 @@ export class CoreLangProvider {
                 return;
             }
 
-            lang = values[2];
+            lang = values[2].replace('_', '-'); // Use the app format instead of Moodle format.
 
             if (!this.customStrings[lang]) {
                 this.customStrings[lang] = {};
@@ -361,6 +363,8 @@ export class CoreLangProvider {
      * @param {string} value String value.
      */
     loadString(langObject: any, lang: string, key: string, value: string): void {
+        lang = lang.replace('_', '-'); // Use the app format instead of Moodle format.
+
         if (this.translate.translations[lang]) {
             // The language is loaded.
             // Store the original value of the string.