change nls_date_format oracle

My oracle now has:

NLS_DATE_FORMAT     DD/MM/RR
NLS_DATE_LANGUAGE   SPANISH

Then I execute:

alter session set nls_date_format = 'DD-MON-RR';
alter session set NLS_DATE_LANGUAGE = 'AMERICAN';

And it changes it without problems, but when I close my sql developer it returns to the configuration in Spanish, how can I modify it forever?

I've tried:

alter system set nls_date_format = 'DD-MON-RR' scope=spfile;

But it's still the same