THIS IS NOT LEGAL ADVICE.
The problems with public domain dedication across jurisdictional lines, as I understand them, are:
1. Users (who are aware of how the law works, in general terms) in a country that doesn't recognize a right to relinquish copyright to the public domain will be afraid of getting sued, so they won't use the software. It might as well not be open source for them.
2. Contributors to your public domain software from those countries are not allowed to relinquish copyright in their works to the public domain, and as a result you're actually accepting work under copyright into your otherwise-public-domain software, which means it's not really public domain software any longer, because of mutual copyright observance agreements between those countries. Thus, for instance, because the US recognizes German copyrights, and Germany doesn't allow dedication of copyrightable works to the public domain, a German contributing code to a public domain work in the US actually contaminates the public domain work with code covered by copyright, thus making it no longer a (pure) public domain work, even in the US, because of US recognition of German copyrights.
If the German in the above example actually transfers the copyright to you, then YOU dedicate the work to the public domain, that might work. I'm just guessing, though, and I'm pretty sure it would require some explicit legally binding (and provable) statements about transferring copyright to you.
But again I feel like that's acting and being scared without any evidence. (And it seems really silly to me that an author would actually have to adopt to the country of the user).
There is very little official statements to find on the internet. All I've ever heard about is these big corporations taking nasty fights, and that seems to scare people with their little tinker projects to death.
Realistically, I would be surprised if e.g. the MIT license was actually more user-friendly than the WTFPL.
I'd love to hear about these thoughts from actual lawyers!