--- a/toolkit/components/extensions/Extension.jsm
+++ b/toolkit/components/extensions/Extension.jsm
@@ -1627,16 +1627,17 @@ this.Extension = class extends Extension
     return this._optionalOrigins;
   }
 };
 
 this.Langpack = class extends ExtensionData {
   constructor(addonData, startupReason) {
     super(addonData.resourceURI);
     this.startupData = addonData.startupData;
+    this.manifestCacheKey = [addonData.id, addonData.version];
   }
 
   static getBootstrapScope(id, file) {
     return new LangpackBootstrapScope();
   }
 
   async promiseLocales(locale) {
     let locales = await StartupCache.locales
@@ -1648,20 +1649,16 @@ this.Langpack = class extends ExtensionD
   readLocaleFile(locale) {
     return StartupCache.locales.get([this.id, this.version, locale],
                                     () => super.readLocaleFile(locale))
       .then(result => {
         this.localeData.messages.set(locale, result);
       });
   }
 
-  get manifestCacheKey() {
-    return [this.id, this.version, Services.locale.getAppLocaleAsLangTag()];
-  }
-
   async _parseManifest() {
     let data = await super.parseManifest();
 
     const productCodeName = AppConstants.MOZ_BUILD_APP.replace("/", "-");
 
     // The result path looks like this:
     //   Firefox - `langpack-pl-browser`
     //   Fennec - `langpack-pl-mobile-android`