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In the winter of 2012 I decided that I would make learning a second language my big project for 2013. I could have picked Spanish without much thought. It is ubiquitous in Texas. It would be useful in the construction, restaurant, or agricultural businesses. Knowledge of Spanish would make those Spanish TV channels seem more interesting. The easy choice is not always the best choice though, so I spent some time thinking about what language(s) would be the best choice. Most online discussions focused on three themes: difficulty, economics, and population.
Difficulty is a negative argument. "Why would anyone try to learn X? It is ridiculously hard!" Every language has its advantages and disadvantages, and yes, some languages are objectively harder than others. The writing systems of China and Japan offer a serious challenge to their learners, but children keep learning those languages. If a student perseveres, he will learn. Difficulty can be a useful factor for ranking languages, but it should not be a decisive factor.
Some proponents argue that business conditions benefit certain languages. Chinese is becoming more popular as the Chinese economy grows. I find this argument unconvincing. For an adult to learn a language requires a significant investment of time and energy. If you compare the opportunity cost of lost business activity (while you learn a new language) to the cost of hiring translators or a bilingual law firm, I think it is very, very hard to justify the effort. In addition, given the critical mass of English in the business world today, foreign businesses will continue to seek workers fluent in English, reducing that incentive.
Many proponents argue that sheer numbers of speakers is a natural benefit for a language. I disagree. If a language has a billion speakers but every one of them holds a world view deeply hostile to my beliefs and way of life, that language holds little interest for me. Also, if that population is distressingly poor and uneducated, what does learning their language offer me? Simple numbers are not an advantage, but large numbers may represent an opportunity to find something interesting.
Benny Lewis has taken the other side of the population argument: it does not matter how many speakers there are for a language since you will never have time to talk to all of them. I agree with that on the surface, but I have another view. I read much more quickly than I speak. I can read more in a day than I can speak in a month. It matters less to me how many speakers there are in a language compared to how much written material is available. Also, in any language besides Spanish, the bulk of my early contact with the language will be through internet, print publishing, and video. Availability of media is where population and economics matters.
A large middle class population supports an environment where print and video materials are created and consumed. Bengali, for example, may have 220 million speakers, but with most of them working in agriculture and garment industry jobs, how many of them have the time or leisure to write the next great novel or produce the next great film? Although Bengali does have a literate history and a Nobel laureate, the current literacy rate below 57% suggests that much of that literature is written by elites for elites. A large, literate middle class promotes more variety in literature and cinema. It may also create business opportunities, but that is just a bonus.
Culture is a large factor in creating such a middle class. A strong work ethic leads to personal prosperity and a positive approach to life. (Found riches such as Arabian oil are less beneficial.) European Protestants have long believed that their religion informs their wordly acts to create progress "on Earth as it is in Heaven." Imperial systems in Persia, China, and Japan have also contributed to individual efforts to achieve recognition. On the other hand, some systems undermine any attempt towards change in order to maintain social stasis. A confident culture can afford intellectual tolerance and exploration. The hostility of Islam and the old Communist states towards disruptive ideas crippled their ability to innovate. Russia has evolved and China is trying to manage its own mutation. Where are the Islamic states heading? In contrast, the tolerance of modern Europeans for alien concepts lets them pick and choose among the best the world has to offer. Japan's partial embrace of western ideas has let that country twice transform itself and shake the world.
In order to maintain a large middle class population, a language needs to have positive demographic trends. Mild population growth is healthy. Overheated population growth is a problem; excess growth in northern India, the Middle East, and Nigeria could create situations of high unemployment leading to social disasters. Negative population trends are worse. The poorer countries of Eastern Europe are losing population due to emigration. The richer countries of Europe and East Asia tend to have falling birth rates. Japan's population could fall by half by 2050. Both situations will undermine the long term health of a language.
Political diversity is a benefit. If an individual annoys one government, it is nice to have somewhere else to go. For the former colonial powers, this can also correct demographic problems. If the population of Spain dips, for example, growth in Mexico can balance it. (Sadly, Siberia fails to help Russia in either way.) This can also provide some cultural variety. French is more interesting due to the juxtaposition of middle class French and Canadians with Francophone Arabs and Africans. The languages of Hungary, Iran, and Thailand are less interesting for being spoken predominantly in a single country. Political diversity may matter less if the surrounding culture is stable and tolerant: Germany seems to accommodate Turks and other immigrants quite well.
In the table below, the DLI column refers to the length in weeks of the basic course offered by the Defense Language Institute. The DLI number serves as a proxy for relative difficulty. The population number is an estimate of the middle class population of speakers with an average per capita GDP >= $20K per year. The Efficiency column is that population divided by the DLI number. More detail can be found here.
There are perhaps thirty languages worth considering.
English is a Germanic language with extensive vocabulary additions from French. This gives the Germanic and Romance languages a large advantage because they will be easier to learn. The Romance languages have more cultural distance and interest. Among the Romance languages, Spanish has a larger middle class population than Portuguese, French, and Italian combined. The populations of all four are larger than that of English. There is a network effect for Romance languages: shared grammar and vocabulary means that a student of one Romance language has a large head start on any other Romance language.
Despite the size advantage of Spanish, French is a strong contender. The French have a peculiar attraction. "The French are so … well, French, and therefore designed by God to seem as provokingly dissimilar from the British as possible." - Julian Barnes. Paul Ward's comment to Français non plus? resonates very strongly with me: "the great joy and mystery for me in my pursuit of French language skills has been in discovering a people who look so much like others in the West, but who have such particular perspectives that I'm constantly being brought up short in my understanding. Learning French isn't about the number of people speaking it, it's about the sheer volume of insights into history, culture, and semiotics that come along with learning it." The country and culture of France attracts more tourists than any other country in the world, but French is also one of the great bridge languages linking Europe, Arabia, and Africa.
German is a contender due to the size and enduring health of the German economy, but the Germanic languages are much smaller than the Romance languages. Dutch could have been interesting as a bridge language but Dutch has almost disappeared in Indonesia and Afrikaans is losing ground in South Africa. The Slavic languages have poor demographic trends. Greece has a small population. The African and South Asian languages have terrible per capita GDP. Also, many of their countries have retained English as an official language which undercuts the need to learn any local language. Hebrew has a small population but it may have religious interest for Christians.
None of the Islamic languages are strong candidates due to the conformist aspect of their cultures. David Tresilian described a trend of "the growing intolerance of literary expression generally, which has made what was always perhaps a minority activity into one that is now that of a sometimes embattled minority. Religious conservatism tends not to value literature on the liberal model – literature, in other words, that carves out a space for intellectual exploration and freedom of expression ..." This trend also applies to candidates Turkish, Persian, and Malay. Internal conflicts between Sunni and Shia and external conflicts with Israel and India make southwest Asia a risky place to work or visit. Malaysia and Indonesia have intermittent periods of religious oppression, often tied to racial conflicts or business interests.
Outside of Europe, the most interesting languages come from East Asia. China and Japan have very old, literate cultures paired with large national economies. Korea, Thailand, and Vietnam are smaller in population, economy, and literate history.
Chinese has variety. There is a large, poor population in mainland China that has been picking itself up by its bootstraps to make China the second largest national economy in the world. There are also rich trading and manufacturing states in Taiwan, Singapore, and Hong Kong. However, all of these lands are either directly under the control of the PRC government or are highly dependent on mainland trade. There is a large degree of political risk involved. To avoid this threat, one could interact with Chinese communities in other countries, but Mandarin is not (yet) the dominant street language of overseas Chinese. Chinese literature is also weaker than one might expect. Chinese authors wrote in an archaic form until 1910, then endured the chaos of the Chinese Civil War and the Japanese invasion, followed by the Cultural Revolution on the mainland and martial law in Taiwan. Hong Kong has been relatively free since World War II, and it has a strong cinema production, but its dominant language is Cantonese.
Japan is the third largest national economy in the world despite a twenty-year economic slump. It has a very homogenous population with European living standards. Japan has a freer culture than mainland China. Since the Japanese urban population achieved literacy in the 1700s, Japan has produced a much wider variety of literature and cinema than the rest of East Asia. The pressing problem with Japan is its impending demographic collapse: the pool of Japanese speakers will shrink and age steadily over the next forty years.
When comparing Asian languages, the type of difficulty presented by a language could be a factor. If a student hates grammar, Japanese, Korean, and Vietnamese are poor candidates. If a student cannot learn to hear or speak tones, Chinese, Thai, and Vietnamese are out of the question. What about overall difficulty? Among the most spoken languages, Japanese and Korean are notable as languages with very few non-native speakers. For some comparisons, consider sinosplice.com, sinoglot.com, and learnlangs.com.
To sum up, the colonial Romance languages combine ease of acquisition, geographic distribution, and European tolerance. Spanish has a larger middle class population and French has stronger literature and cinema. German is also a reasonable choice due to its economic strength and tolerant culture. Outside of Europe, Chinese and Japanese are the most interesting choices, with fascinating cultures and strong economies that offset increased difficulty and political or demographic problems.