It's fairly easy to narrow the choice of languages for "major national security challenges".

I think so because I do not consider any conflict as a major national security challenge unless it involves nukes or a country that neighbours the own alliance. Nuclear conflict requires no soldier to speak a foreign language.

Therefore I still recommend quite the same basic languages as years ago:

basic:
English - NATO language, understood in much of Africa and elsewhere
Russian - understood at Eastern European NATO frontier
Arabic - Southern European NATO frontier

bonus:
Farsi - SE European NATO frontier
Spanish - Latin America (American NATO frontier)
French - understood in much of Africa (and the French are often poor at English)


The Chinese and Indians can often communicate in English.

About 2,000 words each are a good start. That's 6,000+6,000 words, and you can easily learn ten words a day during breakfast (and two hours weekly for learning how to say them). Two years for basic, two more years for bonus languages.

Americans, Englishmen already know English, the Spanish (and pretty much the Portuguese) as well as many others already know Spanish.

Correct writing in foreign language isn't essential and deserves no emphasis in military language training.