Per default all views of IdentityServer3 are not localized and in English. This solution provides a LocalizedViewLoader that can be used to load all views with translations. This implementation is based on the FileSystemWithEmbeddedFallbackViewLoader but with translation capabilities.

Build the solution and include the resulting assembly. Place the directories assets and resources into the root directory of your application. Be sure to set Copy to Output Directory to Copy if newer. All files are loaded into an internal cache after startup, so restart after changing views or language files.

var factory = new IdentityServerServiceFactory();
factory.ViewService = new Registration<IViewService>(
    resolver => new DefaultViewService(embeddedViewServiceConfig, new LocalizedViewLoader()));

The LocalizedViewLoader takes a paramter directory to override the default view directory which is assets.

We use PO files as language files. The name of the file defines the culture:

de.po -> German
de-AT -> German, Austria

If the requested language could not be found, the culture en will be searched, so be sure that this one exists.

For further information how IdentityServer3 works, see Customizing Views.