Not sure that's possible, at least nowhere in the UI.

Seems like you'd have to write a small job to loop through your pages and delete that specific language branch.

There are "delete language" methods in both the EPiServer.DataFactory and EPiServer.DataAbstraction.ContentStore namespaces, but they all take a PageReference or ContentReference as parameter, meaning they will only delete the language for the specified page, not all pages in the content tree.

E.g. 

EPiServer.DataFactory.DeleteLanguageBranch: http://world.episerver.com/Documentation/Class-library/?documentId=cms/7/c8ac5c2b-ff67-869d-e6f3-8703e864f387

EPiServer.DataAbstraction.ContentStore.DeleteLanguage: http://world.episerver.com/Documentation/Class-library/?documentId=cms/7/0feec2ec-d9b3-39bb-4bb6-40f8f8a791d6

Just for reference, I'm posting the updated solution by Adnan here:

http://www.adnanzameer.com/2014/09/delete-language-branch-from-all-pages.html

The following code seems to work for me... Easy enough to wrap in an admin plugin.

var pages = _contentRepository.GetDescendents(ContentReference.RootPage);
var datafactory = new DataFactory();
foreach (var page in pages)
{
	try
	{
		datafactory.DeleteLanguageBranch(page, language, AccessLevel.Delete);
	}
	catch (SqlException)
	{
		// Ignore whether the page exists in the language or not
	}
}