I think that buddhism is losing to the powers of evolution. This means that the power of a population is always stronger than any existing and very feeble Sangha.
I think you underestimate the spiritual power of Buddhist teachers. A sangha which consisted of only a handful of people and a great Buddhist master like Garchen Rinpoche sitting in a warehouse together would have more spiritual power than a vast international bureaucracy.
Does it matter if one is outnumbered by non-Buddhists when one is the student of a great Buddhist teacher? My opinion is "not really."
Is there any truly international Sangha?
I assume by this you mean an ethical sangha. I won't talk about controversial organizations here.
The FPMT seems like an international ethical sangha which even has online courses. Thich Nhat Hanh's sanghas are also quite international, I think.
A Sangha that is independent of the power of a particular human population?
Well, it depends what you mean. Most sanghas of the world exist in modern nation-states and are therefore technically under the power of those governments. But I think in many places, the majority of the local population is simply indifferent about Buddhism, so they don't interfere in any way with the sanghas' activities.
Your question seems to spring from a fundamental distrust of human nature. However, the Buddhist viewpoint is that all people's natural state of being is inherent goodness.
Anywhere a sangha exists its members are still subject to cause and effect and the laws of karma. Enlightenment is the only "escape." Any sort of political, economic, or regulatory freedom is merely a temporary phenomenon.
Ordinary people who ignore karma are feeble. Great Buddhist teachers are mighty, even in small numbers.