Something

ALTER PROCEDURE [dbo].[sp2_GetAllCategories] 
	@SortExpression varchar(50) = NULL,
	@SortDirection varchar (5) = NULL	
AS
BEGIN
	SET NOCOUNT ON;

	DECLARE @SortExpr varchar(50)
	SET @SortExpr = 
		CASE WHEN @SortExpression IS NULL THEN 'CategoryNameEN'
			WHEN @SortExpression = 'CategoryNameEN' THEN 'CategoryNameEN'
			-- WHEN @SortExpression = 'CategoryDescriptionEN' THEN 'CategoryNameEN'
			WHEN @SortExpression = 'IsSubcategory' THEN 'IsSubcategory'
			WHEN @SortExpression = 'CategoryPkey' THEN 'CategoryPkey'
		END		

	DECLARE @sql varchar(Max)
	SET @sql = 'SELECT * FROM Categories ORDER BY ' + @SortExpr + ' ' +  ISNULL(@SortDirection, 'ASC')  
	print (@sql)
	EXEC (@sql)    

END