How to correct SQL-Server installation with wrong language?
This can be done from the SSMS GUI.  Right click on the instance in object explorer and go to properties.

From there navigate to the Advanced page.

There is a drop down item there for Default Language.

Alternatively, if you know the language code you can do it via tsql

EXEC sp_configure 'default language', 0 ; 
GO  
RECONFIGURE ;  
GO


where 0  is English in this case.

See https://msdn.microsoft.com/en-GB/library/ms190682.aspx

You may also need to update or change the default language of logins via their properties page.