With the coming collapse, and likely invasion by a foreign force, what language(s) should we be learning?

If you've not seen it yet, there have been multiple moves over the past few years that will lead to a direct softening of our military.
When combined with the controlled collapse of our economy, this spells certain disaster with respect to a pending invasion being resisted.
China already has plans, leaked from a defector, for how to storm our west coast and work inward from there. Their plan includes concealing troops and equipment in shipping containers on normal trade vessels. They'll have all major west coast docks under control within the first day.
Russia would be an obvious ally to them in this effort.
All of South America has produced a slow invasion over the past decade or two.

Back in college, it was assumed that Japan would become the world leader in business, and it would be pertinent to learn a bit of Japanese to stay competitive.
I went only as far as to buy an English to Japanese dictionary, and that was about it. Turned out that trend wasn't as prophetic as had been proposed...
But what do we expect the next leading language of the new world order will be when the US finally falls?
If you live, what should you learn to speak to get along in the new normal?

I'm reminded of the short-lived TV series "Firefly". In that world, a new language had emerged which meshed English and Mandarin, due to China's influence back on Earth and being the other superpower along with the US that came together to get humanity off of the dying planet and into the stars.