change session nls_language within forms on runtime

Dear all;

how to make forms application multy-languages

i mean if you chose the english language  from a list on the form

the errors and the messages will be in English
and if you chose another language the errors and message will be in the same language;

this statement  works well within sqlplus
(alter session set nls_language=Arabic).
i put it in trigger pre_form
{   forms_ddl('alter session set nls_language=Arabic')   }
but it dose not  work
how i can make it work within forms;

thanks for all.
waiting for replay