No problem.
I will follow up, as promised, with a patch to refactor the internationalization of the info/warning/error messages.

Yes, sounds good.
The current patch is quite aligned with what the existing code is doing, so no modification should be required. This patch could then be used as-is.

Internationalize strings.

A few additional changes based on Nate's comments.

No, nothing bad will happen. The importer will try to create a subfolder if needed. If it somehow doesn't succeed, it will result in a FileUtils::RenameFailed.
In that case the file will simply be skipped altogether. It will remain untouched on the source device, and it will also not be deleted if the user tells the importer to delete the imported files. Deleting is only done for successfully imported files.
I'm already working on the other patch. Luckily, it seems like it might be relatively simple to implement some basic UI warnings for failed items.

For traceability's sake, I would separate the proper error handling into a separate patch/commit.
It's completely missing at the moment, so it seems strange to mix it in with a "new feature".

Register as FileUtils::RenameFailed if the subfolder cannot be created.

You're absolutely right that there needs to be a GUI warning/report of some kind. However, the current importer does not have the infrastructure to handle this: it does nothing with failed imports (of any kind) except print a warning message to stdout. See e.g. the FIXME/warning on lines 208 and 255.

I'll certainly consider it.
Let's see if I can find the time. :-)

Swap keep and delete buttons. This makes keep the default action.

Add unit tests and solve bug when mulitiple files of same name exist.

Use QScopedPointer for QIODevice pointers

The purpose of this dialog box is to ask the user what should be done with the original files on the source device that the photos were imported from. I.e. deleting them on the source device, e.g. the camera or smartphone. Only the photos that were imported successfully are then deleted from the source device (so not the ones that failed or were not selected to be imported).
BTW, the dialog box pops up after the import itself has been done.

Since this is my first ever contribution to KDE: is there anything that I still need to do on my side? Or will this commit be applied without further intervention from me?

Sure, my "git" email address is devriesb@gmail.com .