travel or academics?

Business, probably a programming language like Python.

Socially: where do you live and where do you want to live? Mandarin will become increasingly important give the rise of China. OTOH, if you were working with India, Hindi would be useful.

Travel: where do you like to travel? Europe? French or German could be useful. South America: Portuguese and it would become easy if you already speak Spanish.

Academics: English is number one and everyone else is far behind. German and Chinese and some Japanese seem to lead the way.

English, Spanish and Mandarin are the most useful languages.

In a tier far below:

I'd say Arabic, Russian, Portuguese, German, Japanese and French are of next order of importance.

Third tier would be Hindi, Swahili, Urdu, Korean and Farsi.