When was it decided that only political agendas and ideas other than "right-wing nationalism" are allowed?

Democracy is about including all kinds of speech, including "right-wing nationalism" and "left-wing cosmopolitanism" or what have you.

If non-right-wing-nationalists legitimize the banning of right-wing-nationalist speech, then if right wing nationalists come to fashion/power they'd be justified to ban left-wing/progressive speech too.

That's not exactly a democratic path.