but has no legal value in France for example as you can't reject all your rights.

It has disputed legal effect in the US, too. But people (individuals and businesses) in practice don't seem to be concerned by it -- in either jurisdiction, or most others -- enough to not use software that is dedicated to the public domain like SQLite, so as a creator if your concern is signalling to people that it is okay to use it, a PD dedication seems likely to be clean.

And any license text is unlikely to have been vetted by lawyers familiar with the legal system of every foreign jurisdiction where it might be applied, so you probably have the same problem with them (and the more complex the terms are, the more likely that they have some unforeseen problem in an unconsidered jurisdiction.)

Is it possible to declare something public domain, and license it as a "fallback"?

That's the effect of the CC0 license: dedicate the work to the public domain, if that isn't possible give away as many rights as possible

https://creativecommons.org/publicdomain/zero/1.0/deed