I would like to note that France has what the local law calls association loi 1901, which is very advantageous for a open source projects. In short it is an entity that can own stuff, accept donation, but cannot do for profit operations. Projects such as VLC and ffmpeg are using this. The requirement are very lax (you basically need to have an address somewhere in France for administrative bullshit) and have at least 3 founders. They don't need to be French or live in France, in fact, last time I checked, you could even do one as an illegal emigrant, which I found funny. I had a long chat with Jena Baptiste Kempf who created VLC about this for the last big open source project, but we ended up going for a structure in HK because we had Chinese users and donors and that was easier administratively this way. Chinese special case excluded, this was by far the best option we had.

501(c)(3) is extremely easy to set up.