on double taxation treaties, where everything else does not apply ( no employer, no center of life, no family, etc,) the last resort for taxation is not the nationality, it is the passport that the individual hold.
Social charges are due where your employment takes place ( except for short terms, where it is due at the headquarter's place)
So, let's say you have a german citizenship, employed by a portuguese company, but leaving in France with your family, and taking your rest there.
The said company will have to pay the social contributions in France, while retaining the right to have your income taxes withheld in Portugal. The double taxation treaty kicks in, and the if the taxes paid in Portugal are representative (read 2/3) of what would have been paid in France, no income tax is due in France, and your benefits ( retirement, loss of employment, medical coverage, etc.) are paid and taken where you live. ( which make sense) Nethertheless a tax return form has to be filled in, and will come back with a figure showing zero. If the state decides not to tax you in Portugal, you will have to pay full taxes in France.
Now you are the same german national, paid offshore, to an offshore account, with no family, leaving from hotels to hotels all around the world, not spending more than 25% of your time at any place on earth, employed with the typical contract found now everywhere (BVI, etc.) This payment is gross/net, whatever. You are liable for your income taxes somewhere, and this is Germany.
Do not quote me wrong, this is the law, it is very unlikely that someone from the tax office will catch you if you do not pay your taxes in Germany. But this is the bottom line, where everything starts from.