Fwd: concatenate and use as field

Hi,

i have a character varying variable and i concatenate with some other variable, using the '||' operator.
the result of this concatenation should be the name of a column in my table.

however i don't know how to tell that this new concatenated string is a column name.
how to do it ?
thanks.

in fact i'm trying to do the following thing:

select id, theme_ || $1 from themes;

and based on the parameter my stored procedure has received, it should create something like that:
select id, theme_eng from themes;

if $1 = 'eng'

I think i'm a good way, but i still have a problem with the performance.
when i call my stored procedure, it can take 0.1 s to 3.5 seconds to execute it on local computer.
How can i improve it ?

here is my stored procedure:
CREATE OR REPLACE FUNCTION sewe.get_category_and_amount(character varying)
  RETURNS SETOF category_amount AS
$BODY$
    DECLARE
        inLanguage ALIAS FOR $1;
        outCategoryAndAmount category_amount;

        Lang character varying :='';
    BEGIN
        IF inLanguage = null OR inLanguage = '' THEN
            Lang := 'eng';
        ELSE
            Lang := inLanguage;
        END IF;

        FOR outCategoryAndAmount IN
            EXECUTE 'SELECT id, theme_name_' || Lang || ' FROM themes WHERE parent_theme IS NULL ORDER BY theme_name_' || Lang || ' ASC'
        LOOP
            RETURN NEXT outCategoryAndAmount;
        END LOOP;
    END;
$BODY$
  LANGUAGE 'plpgsql' VOLATILE
  COST 100
  ROWS 1000;