I originally posted this over on SWJ's Facebook thread, but I see the conversation is far livelier here:

Hindi and Urdu are actually the same language--Hindustani. They essentially have different scripts and some different vocabulary.

To the larger question, guessing where we'll be in 20 years is a fool's errand. There will doubtlessly be cause to learn any number of local languages. The best option, then, is to learn lingua francas: Modern Standard Arabic, Spanish, Russian, French, Swahili, Hausa, Persian, Hindustani, Mandarin, etc. You can narrow this list by (1) what regions interest you, (2) where you think US strategic interests lie, and (3) where instability is likely to occur.

In the near term, I'm a fan of Arabic, Somali, and Urdu. In the long term, Mandarin.

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A few responses to what people have posted on this thread--

Fuchs - I think you vastly oversimplify the process of learning foreign vocabulary. 10 words/day is VERY fast over the long term. If you're not ACTIVELY using these words, you will lose most of them, and it's likely your brain will not appropriately acquire the rest. (I.e. your brain will associate them with their English translation, vice their actual meaning.)

I also take issue with your assertion that writing deserves no emphasis in military training. I agree that verbal communication is paramount (and not just in military application) but knowledge of the script can be necessary--especially for personnel who have any dealings with official documents like IDs or who need to be able to identify propaganda materials. This means even small unit leaders--or even PFCs--could benefit from being able to read the script. Also, personnel at higher levels benefit from having a clue about the script, especially for languages where transliteration is not standardized. Finally, unless we're talking about Chinese, scripts are relatively easy to learn.

I guess I'm piling on Fuchs here, but the Indonesia comment is way off base for a few reasons. I'll just point out the most obvious one: the Strait of Malacca.

Abu Suleyman - I would think that most Chinese companies that do business in the Middle East will use English as a common language.