Esperanto is a constructed language that has been proposed as a possible global language. I’m personally an avid supporter of the idea. It is very logical and easy to learn. It can probably be learned by most people in just a fifth of the time it takes to learn English.
I must admit, I was first drawn to the idea because, as a native English speaker, I feel a bit guilty that I can travel the world and expect people in other countries to know my native language while I get to remain monolingual. I first heard about Esperanto about a year and half ago, and the more I study and learn about it, the more enthusiastic I become. I just study it here-and-there in my free time and have become proficient, if not quite fluent, in it.
It was developed in the 1880s by a Polish man named L. L. Zamenhof. This was the euphoric, post Darwin and Marx decades when Europe seemed to be giddy with social-engineering projects. It was initially a very popular Idea. In 1922 it even missed out by a single vote on becoming the working-language of the League of Nations. Many governments proposed that it be taught in schools as a universal auxiliary language. Sadly it lost momentum during the nationalistic, anti-globalist fervor of the WWII era. But it has seen a resurgence with the internet.
There are large online communities of Esperantists (as they’re known), and you can find it on Duolingo and Google Translate. It also has a larger Wikipedia than most national languages. All-and-all it currently has probably more than 2 million speakers and at least 2 thousand native speakers. Thousands of books have been written in Esperanto. It also has its own couch-surfing-like service, in which Esperantists can pretty much travel the world accommodation-free, crashing at the homes of other Esperantists.
It isn’t without its problems. For one, it’s seriously Eurocentric, with the vast majority of its root words coming from Romance and Germanic languages and some from Greek or Slavic languages but none from outside the Indo-European family unless they happen to be loan words that are common in European languages. Another valid criticism was eloquently laid out by famed writer and linguist J.R.R. Tolkien (who was himself once an enthusiastic Esperantist) when he said that Esperanto and other constructed languages are “dead, far deader than unused ancient languages, because their authors never invented any Esperanto legends.” You could argue that a language needs a base culture on which to thrive, but I think the Internet may be providing that in lieu of legends.
Esperanto uses a logical system of affixes, in which words can be built form parts. This makes the vocabulary you have to learn much smaller than in natural languages. It’s grammar is completely logical and regular.
There have been studies that suggest that Learning Esperanto first makes it easier to learn other foreign languages later. Here is a TED Talk that explains this well. For example, there was an experiment done in Manchester in which one group of students learned Esperanto for one year and French for two, and another group studied French for all three years. The first group scored better on French exams. The reason for this is (I think) is because the easy and fast acquisition of Esperanto teaches you how grammar can be constructed in ways that are different from your native language. In other words, learning Esperanto teaches you how to learn languages by building your confidence and providing you with a base knowledge of how foreign grammar works, whereas learners of natural languages get discouraged after devoting large amounts of their time and seeing little improvement.