I can comment on #3 at least - for that, .txt is indeed the most universal format, though XML is also supported on Palm / WM in 2.0 and will be supported on iPhone and any other platforms we branch out to (desktops etc). But XML's a lot harder to create files in, and for basic vocabulary lists at least it doesn't really add any extra features.
For #2, we create our flashcard lists using a regular old text editor (the fast, Chinese- and large-file-friendly
EmEditor for Windows - sadly there's nothing even remotely comparable to it on Mac) - you could also lay them out in a spreadsheet like Excel, but that can sometimes distort parts of the file in awkward ways, so I'd recommend keeping things in plain text as much as possible.
If you already have a copy of Pleco on another platform, or have access to a friend's copy, one thing you could try that might get you well on your way to a translated version of the NPCR flashcards is this. First, make sure you've got the latest version of the free HanDeDict Chinese-German dictionary installed in Pleco. Then, import the NPCR list into Pleco's flashcard system with HanDeDict set as the main / only dictionary to match entries to. To do this, tap on the "Dicts..." button in the Pleco flashcard "Import" dialog, select "Prefer Dicts" for "Definition Source" at the top of the screen, and remove everything but HanDeDict from the dictionary list on the left side of the screen. Also, in the main "Import" screen, set "Duplicate Entries" to "Allow." Proceed with importing the NPCR flashcards, then turn right around and export them to a text file via the "Export" screen, making sure to choose "Text File" as your file format and to check the "Card definitions" and "Free dict defns" boxes.
After doing all of that, that exported text file should be a copy of the NPCR list, but with every word in it that has a HanDeDict entry getting its definition from that instead of from the source list. You can then clean up / correct those HanDeDict-supplied definitions and put in your own translations for the ones that are still in English.