Issue: Batch create translations

You can batch add existing pages to a category from within the category. Use Add Listing > Add Existing Page. Then, use the filters to sort by language. This modal allows you to select up to 200 pages at one time to put into the category.

Unfortunately thats not useful at all when you have thousands of pages. Also: it seems there are no filters but only sorting options. There is also no function to select all.
If the interface would have multiple filters that can be applied (show all English page in category X) and would allow us to select all results, then it would be useful. The interface is too small to really work with it when you have many results. It needs a dedicated page instead of an overlay.

They aren't sorting options. The results are always sorted alphabetically. When you are only showing results from Category Y, that is a filter. Regarding selecting 2 filters at once, this is something I have considered (e.g all pages that appear in both Area X and Category Y). I didn't think it was worth doing at the time because of how rarely I filtered by anything other than the first 2-3 characters of a title, and it seems like there are plenty of more useful features that need doing.

But now that someone (you) has requested it, it's more than just a nice-to-have idea in the back of my mind.

Now that you mention it, a Select/Deselect All Shown would be very handy when I make the vulnerability pages. For example, when I made An Unconfirmed Vulnerability, I searched "4.0" and then had to click close to 200 boxes. While it was much faster than adding the category manually by editing all 200 pages (the reason the overlay even exists), it still left something to be desired. Shift+Click support would be useful too.

However, I'm struggling to understand how you want the various functions in your OP to work. The third one, batch add translated pages to the respective translated categories, makes sense to me.

But all the other ones seem like you are asking to batch create placeholder translations of pages meeting specified criteria, or batch creating placeholder translations of categories meeting criteria. I say placeholder, because in a batch process, there would be no means for anyone to actually translate any of the body content. It would be the old content with the wrong language selected. The only thing that would get done is the titles, but also with placeholder text like "Original Title (ISO-code)". Because of this, I'm not really seeing the usefulness of these functions.

For the batch creation of translations this would create stub pages.
It would add structure. The third function cannot be performed unless there are translations of all pages. Otherwise it would not be possible to add all these translations to the correct category,
It makes sense for adding structure. For some sites there would be a downside that a bulk of stub pages are created. But if it is certain that all pages will be written anyway, then it makes perfect sense.

If this gets implemented I will request a few other batch features which will be useful for a lot of admins. But before those bulk actions could be performed we first need the pages to exist.

Wanted to mention that there is a checkbox for Select All Results / De-Select All Results in the Add Existing Page dialog starting in 4.0.18. It just made my job creating vulnerability pages significantly faster, so thanks for that suggestion. I forgot that I mentioned Shift+Click support; I will see if we can add that too. I realize that is only a small part of this request, but I thought I would mention what progress has been made.