Lack of some translations in 5.1.0

[rafsaf]

Hi!

Currently in 5.1.0 in new django_registration/activation_form.html after https://github.com/ubernostrum/django-registration/pull/247 there are some translations missing, for example:

At first glance:

https://github.com/ubernostrum/django-registration/blob/a397174f7149c6f038fb68f09ba4d46064be8281/src/django_registration/backends/activation/forms.py#L26-L29

I think this activation_key needs "label" label=_("Activation key") and beside that some "makemessages", "compilemessages" commands are needed because both error messages don't appear in any .po files. I can craft some PR in spare time.

But there are quite many languages supported here so maybe instead of bump each of them by individual developers (which looks to be the case in the past when i look on code base) maybe some more general solution is possible (AI generated translation or something, there are quite good nowadays).

BR