tablename as attribute in pgplsql

DECLARE  usertablename ALIAS FOR $1 ;  userid ALIAS FOR $2 ;
  id_lang BIGINT ;
  var VARCHAR(200) ;

BEGIN     var := 'client' ;
     SELECT id_language INTO id_lang FROM client WHERE id_user = userid ;     IF NOT (id_lang = 1 OR id_lang = 3) THEN
      id_lang = 1 ;     END IF ;
 
     RETURN id_lang ;

END ;

I want to use client table as a variable

How could I do