Tip: Are you a non-native English speaker? I have just finished creating a 
Web AppStarting to learn a new language is a big decision. It requires a lot of dedication and perseverance, and it is a decision that can completely change the rest of your life. You should base your decision on both objective and subjective criteria (that is, criteria that only apply to you, personally). It is impossible to list all criteria relevant to all readers of this article, but I will try to discuss at least the most important ones.
Subjective reasons to learn a language
I will discuss the subjective criteria first, as they may be actually more important than the objective ones:
Motivation
If you don’t feel motivated to learn, it will be a slow process. When you don’t enjoy what you are doing, your brain will find a way to avoid the activity as much as possible and gain as little as possible from the time you invest in it—do you know those moments when you’ve read a page in a textbook only to realize you have no idea what you have just read?
Enjoyment brought by learning a particular language may be the single most important factor when deciding which one to learn. If you don’t feel motivated to learn a particular language, you will likely stop before being able to perceive any potential benefits.
Intentions
You should ask yourself: ”Why do I want to learn a second language?” If your aim is to study early scientific or religious works, Latin would be your language of choice. If you want to be able to speak with your distant relatives from a different country, you may want to learn their language. If you want to move to another country for whatever reason, it is definitely a good idea to learn the language(s) spoken there.
In other words, it’s no good if you learn, say, Chinese based on the objective criteria described below only to find out that you don’t use it at all because all you really wanted was to be able to speak to your friends, most of whom happened to be Polish. So, think carefully whether there is a reason why you prefer a particular language to another.
Character and culture
I had a very interesting discussion under my article about usefulness of learning Esperanto related to its social aspect. It seems that the Esperanto community is extraordinarily welcoming and social. If you intend to travel a lot and enjoy making friends in an extremely diverse (even though relatively small) community of people, Esperanto may be the right choice for you. If you, on the other hand, prefer communication mainly with people with similar interests or don’t travel much, you may prefer to choose another language.
More generally, if you particularly like something about the culture of the speakers of your potential target language, or if it has some other appealing quality (even if it is just that you love the way it sounds or its writing system), it may be a good enough reason to start learning it. This factor is closely related to motivation; if you enjoy learning a language because of its culture, you are more likely to stay motivated.
Objective reasons for learning a language
Even though there’s no way to objectively judge whether one language is “better” or “more useful” than another, there are some facts that may help you decide which one is right for you:
Number and distribution of speakers
Some languages are spoken by just two remaining living speakers, others are spoken by billions. Unless you learn a language for academic reasons, you may want to learn a language you are most likely to use; not only face to face but also on the Internet and to read literary works.
Mandarin Chinese and English are both spoken by more than a billion people in the world. A slight difference between the two is that while Mandarin speakers are generally completely fluent (the vast majority of these are native speakers), speakers of English as a second language often struggle to express themselves eloquently.
There’s a wealth of information on the Internet written in Chinese, and there are Chinese minorities in many parts of the world. You will be able to find Chinese restaurants almost everywhere, and this may be a good starting point for engaging in conversation with native speakers.
Spanish is the next most commonly spoken language with about 500 million speakers worldwide. Apart from several hundred million people in Latin America and Spain, it is spoken by about 12% of the US population. There are a lot of Spanish speakers in many European cities; I don’t have any official statistics, but in Berlin (the city where I studied), native Spanish speakers seemed to form the largest group among all foreign students.
French is spoken by about 340 million people, not only in France and Canada but also as a lingua franca in 31 African countries, and it is among the most commonly used languages on the Internet.
Russian is spoken by about 260 million people, and it is a lingua franca in most of the former Eastern Bloc. It is the second most popular language on the Internet (after English).
Portuguese is also spoken by about 260 million people and is used in Brazil, Portugal, and several African countries.
German is spoken by about 150 million people, out of which 100 million speak it as a native language. It is the third most commonly used language on the Internet. It is spoken mostly in Central Europe.
Note that Hindi and Bengali are also spoken by quite a large number of speakers (400 and 250 million, respectively), but they are relatively restricted to certain parts of India and their presence on the Internet is quite low (less than 0.1%).
Also note that Arabic is spoken by about 290 million people; however, “Arabic” refers in fact not to a single language, but rather to a wide range of “dialects”, some of which are not mutually intelligible. There is a standardized form of Arabic called Modern Standard Arabic. Although there are no native speakers of Modern Standard Arabic, most speakers of various Arabic dialects understand it and are able to use it to some degree if necessary.
Ease of learning
Not all languages are equally hard to learn for all people. Whether a new language is hard or easy for you to learn depends mostly on the languages you already speak. I address this topic in a separate article.
Culture and literary works
Not only can the culture of a language make it more appealing to you, but there are some objective factors to consider. There are lots of globally influential works written in just a handful of languages, while there are none in most.
The fact that a work is not “influential” does not mean that it isn’t “good”, and sometimes influential works are considered poorly written by many. For example, in terms of its cultural impact, it doesn’t matter now whether the Bible was well written or not, and scholars generally agree that the quality of other Greek classics was much higher than that of the New Testament. It doesn’t even matter, in terms of influentiality, which parts of it describe actual events and which are fiction; without a working knowledge of its content, it is impossible to fully appreciate most of western classical art and literature.
Reading a translation of a book is never the same as reading it in its original language. When you read an influential work, it is not just a pleasurable experience; it also helps you grow intellectually. Therefore, you may want to choose a “more influential” language rather than one with a small body of literature.
It is impossible to order languages by “influentiality”, but as a rough guide, the following languages (given in alphabetical order) produced a great number of authors influential on a global scale: English, French, German, Italian, Russian, and Spanish.
