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