Cannot delete language if folder has language settings

Description

Unable to delete a language if a folder has language settings.

1. Enable a second language (e.g., Swedish).
2. Open the language settings page for a folder.
3. Replace the folder id with a real ID, such as
http://../EPiServer/CMS/Edit/LanguageSettings.aspx?IsInLegacyWrapper=true&id=folderid
4. Enable the Swedish language on the folder, save it.
5. Open Manage Website Languages and try to delete the language you enabled.

Expected
You get a warning that the language is used on the folder

Actual
Content with id '236' is of type 'EPiServer.Core.ContentFolder' which does not inherit required type 'EPiServer.Core.PageData'