Background tasks, though they are essentially extensions of a app, only have limited capabilities where using the WinRT APIs are concerned. They can access the app’s appdata folders and settings, for example, and perform file I/O with Windows.Storage APIs. No problem there. Background tasks cannot, however, generate UI with the exception of issuing toast notifications and tile update.
Background tasks are also able to perform string lookups in resource files using the Windows.ApplicationModel.Resources.ResourceLoader class or Windows.ApplicationModel.Resources.Core.ResourceManager.current, meaning that they do have access to the app’s localized content.