Is it possible for man to ever have a universal language? no, not really, people are too stubborn for that. However, that presents a question about what a universal language would be like.
Origin
In 1887, the ophthalmologist (which is like an eye doctor), Ludwig Lejzer Zamenhof, created one of the earliest planned languages, Esperanto. The term 'planned language' referring to a language by a single person to be suitable for human communication, so another word for a conlang.
Esperanto is an international auxiliary language (IAL) which means that it was made to provide easy, fast, and/or improved communication for everyone in the world (or a vast majority of the world's population) normally without replacing other languages
Esperanto is the most popular IAL ever invented with some of the highest predictions being two million speakers (that's a 2 with six 0s or 0.025% of the global population). It is also the only conlang that has native speakers, having roughly 1000.
It even has it's own committee of 45 members called The Akademio de Esperanto (AdE), which was proposed by L.L. Zamenhof in Boulogne-sur-Mer at the first World Esperanto Congress. The AdE exists to make sure the evolution of Esperanto is consistent with the Fundamento de Esperanto in alignment with the Declaration of Boulogne.
Writen and ratified in 1905, The Declaration on the Essence of Esperantism (Deklaracio pri la esenco de Esperantism), aka the Declaration of Boulogne (Bulonja Deklaracio), consists of an introduction and five points. The introduction states that the 5 points are important because many people misunderstand the nature of the Esperanto movement. The 5 points includes the following statements (paraphrased):
Esperantism is the endeavour to spread across the entire world the use of a neutral, human language. It is intended to give people the ability to understand eachother regardless of native language
Esperanto is the more realistic IAL and that it must be worked on for the aim of an international language
Esperanto is no one's property and it may be used for any reason
Only the Fundamento de Esperanto has authority over Esperanto, No other single entity, including Zamenhof, has authority over it. If a linguistic matter isn't covered in the Fundamento it is up to the individual how they'd resolve the matter
An Esperantist is a fluent Esperanto speaker. Involvement with the Esperanto community is recommended but not necessary
Language
There are 28 letters in the Esperanto alphabet. From the Latin alphabet it is missing: Q, W, X, and Y. But instead includes variants of the letters C, G, H, J, and S with a circumflex and a u with a breve (which is only used in Esperanto, the 20th Century Belarusian Latin alphabet and formally in the Romanian alphabet). Esperanto has between 5 and 11 vowels: 5 monophthongs (a pure vowels sound with little to no change in articulation from beginning and end) (/a/, /e/, /i/, /o/, /u/) and up to 6 diphthongs (a combination of two adjacent vowels within the same syllable, also known as a gliding vowels) (/ai̯/, /oi̯/, /ui̯/, /ei̯/ and /au̯/, /eu̯/).
Esperanto is primarily formed by stringing together roots, grammatical endings, prefixes and suffixs. Compound words are formed the same as they are formed in English modifier-first head-final (for example birdsong and songbird. Where birdsong is about the song made by birds yet a songbird is the bird that makes the song).
Lingvo Internacia, published by Zamenhof in 1887, contains 900 roots which can be made into thousands of words using compound forms prefixes and suffixs. Later in 1894, Zamenhof released the first Esperanto dictionary, Universala Vortaro, which had even more roots and along with this the rules of the language allow speakers to borrow new roots from other languages where needed, however it is recommended that speakers try to use the most international form.
Words in Esperanto are often calqued meaning that their meanings are coopted from other languages. An example of this is 'muso' (mouse) which came to also mean computer mouse as it has done in many languages (e.g. English mouse, French souris, and Spanish ratón)
Controversy and Criticism
Language-Neutrality Esperanto is very Eurocentric since its primary based on Romance, Germanic, and Slavic Languages. People argue that a true neutral language would include origin from a wider range of languages
Gender-neutrality Esperanto like many of the languages it was based on includes gendered nouns but not in the way French does with Le, La, or Les but instead with the way English does with words with -ess like waiter/waitress or baron/baroness. In Esperanto its a cass of -o or -ino for example patro 'father' or patrino 'mother' and doktoro 'male or otherwise unspecified PhD doctor' or doktorino 'a female PhD doctor.
Grammar Esperanto involves things like grammatical case and adjectival agreement which is just not something everybody uses for example in English you don't normally need to think about what you need to put on an adjective to make it agree with the noun whilst in languages like Spanish, whether you put an -a on the end of trabajador can change who you are talking about.
Alphabet Esperanto uses two diacritics/accents: the circumflex and the breve. This is because the alphabet was invented with the French typewriter in mind however in modern days even though computers support unicode it can be difficult to enter letters with diacritic marks.
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