Also read the plugin files using the content provider. (#12552)

src/Umbraco.Web.BackOffice/DependencyInjection/UmbracoBuilder.LocalizedText.cs

@@ -37,6 +37,8 @@ namespace Umbraco.Extensions
             IFileProvider webFileProvider = webHostEnvironment.WebRootFileProvider;
             IFileProvider contentFileProvider = webHostEnvironment.ContentRootFileProvider;
 
+            IEnumerable<LocalizedTextServiceSupplementaryFileSource> localPluginFileSources = GetPluginLanguageFileSources(contentFileProvider, Cms.Core.Constants.SystemDirectories.AppPlugins, false);
+
             // gets all langs files in /app_plugins real or virtual locations
             IEnumerable<LocalizedTextServiceSupplementaryFileSource> pluginLangFileSources = GetPluginLanguageFileSources(webFileProvider, Cms.Core.Constants.SystemDirectories.AppPlugins, false);
 
@@ -50,7 +52,9 @@ namespace Umbraco.Extensions
                     .SelectMany(x => x.GetFiles("*.user.xml", SearchOption.TopDirectoryOnly))
                     .Select(x => new LocalizedTextServiceSupplementaryFileSource(x, true));
 
-            return pluginLangFileSources
+            return
+                localPluginFileSources
+                .Concat(pluginLangFileSources)
                 .Concat(userLangFileSources);
         }