Fix ugly non-sense strings in signup page

We need badly to (at least temporary) fix strings shown when a user registers.
Now one see unmeaningful string such as "signup.ui.loginData.username" which is clearly not the intended behaviour.
This is a regression of commit 836274ad0aff5131efb5741750aec9583a6e2672 that totally changed the way the l10n is handled. This lead e.g. "signup.ui.loginData.username" to mean nothing (also making it not translatable obviously).
This fix aims at showing decent strings.