昔、MySQLで初めて作った、ユーザー定義関数です。
言語を付け足していけば、拡張できます。RETURNS の CHARSET は必要に応じて変更してください。

CREATE DEFINER=`root`@`localhost` FUNCTION `fc_dofwtxt`(
	`idx` tinyint,
	`langcd` varchar(4)

)
RETURNS varchar(20) CHARSET sjis
LANGUAGE SQL
NOT DETERMINISTIC
CONTAINS SQL
SQL SECURITY DEFINER
COMMENT '言語指定曜日文字列スカラー関数'
begin
  declare res varchar(12);
  
  if (langcd = 'jp') THEN
  	select 
  	  case (idx)
  	  when 2 then '月'
  	  when 3 then '火'
  	  when 4 then '水'
  	  when 5 then '木'
  	  when 6 then '金'
  	  when 7 then '土'
  	  when 1 then '日'
  		end as restxt into res;
  
  END IF;
  
  if (langcd = 'jpp') THEN
  	select 
  	  case (idx)
  	  when 2 then '(月)'
  	  when 3 then '(火)'
  	  when 4 then '(水)'
  	  when 5 then '(木)'
  	  when 6 then '(金)'
  	  when 7 then '(土)'
  	  when 1 then '(日)'
  		end as restxt into res;
  
  END IF; 
	
  if (langcd = 'ens') THEN
  	select 
  	  case (idx)
  	  when 2 then 'Mon'
  	  when 3 then 'Tue'
  	  when 4 then 'Wed'
  	  when 5 then 'Thu'
  	  when 6 then 'Fri'
  	  when 7 then 'Sat'
  	  when 1 then 'Sun'
  		end as restxt into res;
  
  END IF; 
	
  if (langcd = 'cnp') THEN
  	select 
  	  case (idx)
  	  when 2 then '(星期一)'
  	  when 3 then '(星期二)'
  	  when 4 then '(星期三)'
  	  when 5 then '(星期四)'
  	  when 6 then '(星期五)'
  	  when 7 then '(星期六)'
  	  when 1 then '(星期天)'
  		end as restxt into res;
  
  END IF;  		 	
	 
  
  return res;
end