How to Change the Default Language for a Login in SQL Server

To change the default language for the login to a different language:

USE [master]
GO
ALTER LOGIN [John] WITH DEFAULT_LANGUAGE = English
GO

change_default_language_for_login

To change the default language for a login using SQL Server Management Studio (SSMS):
Right click on a login, click ‘Properties’, and choose default language:

change_default_language_for_login_ssms