Huh, that's actually a very good idea, Philippe! The aggregate group system came about because we needed a way to group pages about the same thing, especially like in the case of the IEG (Individuel Editor Grants) pages where there were several interconnected pages and templates, and the templates needed to be translated in order for the pages to be fully translated. So to be able to give a single link to how to translate, aggregate groups were needed. However, one problem that I had with that was that pages could only be part of one aggregate group (I think that has changed now), so when I had a template that was used both for IEG and another set of pages, that had to be treated differently.

But yes, your idea of using categories instead, so that I would be able to give a link to Special:Translate?category=IEG_translatable_pages and have all pages in that category appear in the translate form, is a great one, and would be a very good replacement for the aggregate groups. Maybe you should write a Phabricator request for this feature, so the internationalization team can have a go at it at some point?