Second language for survival skill

[Tarrel]

Depends how you define "Survival". If we're thinking about being marketable in a fast-changing world where the economic centres of gravity are changing, then I'd say Mandarin or Spanish, to give you access to the fast growing South American and Asian job markets. Another possibility is Russian, as this is the "Rosetta Stone" that will give you an understanding of many of the eastern european, balkan and baltic languages. It will also give you an understanding of the Cyrillic alphabet, which in itself is useful.

35 years ago, when I did my business and engineering degree, we all learnt German, as Germany was seen as the economic powerhouse with which we would need to do business. Now, however, I think it's a bit of a blind alley. Influential as Germany is in the EU, the language is not that important on the World stage. likewise French. Traditionally the language of diplomacy, and widely spoken in North Africa, but probably not that useful where the economic action is.

If we're thinking about other aspects of survival, then the question is a trickier one.

Considering bugging out to mainland Europe WTSHTF? probably French or Russian.
Heading to Scandinavia to outrun climate chaos? swedish or Norwegian.

Bear in mind that, whatever language you choose, it will take a long time to become fluent and lose the accent, so don't expect to blend in. However, having just a few words of the language of a country you are visiting, sends a powerful message that you want to be accepted.

HTH.