The Chinese writing system was actually well adapted for this task, because it can be used as the common written form of different local languages with completely different spoken vocabularies. A picture of a house can mean "house", "maison", "domus", casa" etc, etc...

" it [Esperanto] is just about as hard as English for speakers of German or a Slavic language; and it's harder than English for everybody else on Earth" -- definitely untrue, eo is orders of magnitude easier to learn than English for German or Slavic or Chinese speakers - just consider the abysmal English phonology and almost completely divorced orthography, inconsistent germanic vs. francolatin vocabulary, idiosyncratic syntax, complex tenses etc., we could go on and on about the relative merits of eo compared to English, with the sole exception of number of speakers and public acceptance.

@RadovanGarabík: "Inconsistent germanic vs. francolatin vocabulary" of English: Esperanto vocabulary is even more inconsistent, having both Romance and Germanic and in addition Slavic roots. "Complex tenses:" English has fewer tenses than Esperanto. "Abysmal phonology:" compared to what exactly? It's true that the phonology of English varies quit a bit between the different varieties, but that's true for all multicentric languages. So all I can say is pretty please citation needed.

"All children learn their first language with equal ease" - Generally true, but that's for natural languages. I recall reading some years ago of a Klingon enthusiast who raised his son on Klingon. He reported that the kid picked it up, but hated it.

@alephzero: Chinese characters are not pictures and most of them never ever were pictures. Test: does 家 (jiā) really suggest a house to you? And the affirmation that "it can be used as the common written form of different local languages" is plain false. Hint: Chinese, just like English, relies very heavily on word order to supplant its almost complete lack of morphology; word order varies greatly between languages. Second hint: Japanese uses Chinese characters for the stems of words, but must need use phonetic Kana to indicate inflections.

@AlexP eo has a mixed source of vocabulary, but then it continues on being consistent. English has (way too many) word pairs like hand/manual, eye/visual, body/corporal where what would be usually a derived term is an imported one. Way too bad for second language speakers... I can go on about tenses (eo has only three used tenses, the compound ones are little used and syntactically can be reanalysed as not being real tenses), English phonology - don't make me laugh, just see the table of consonant and vowels and allophones. Compare with Italian or Japanese. Anyway, these comments...

@RadovanGarabík: And let's not forget "inconsistent syntax", as opposed to the almost completely undefined syntax of Esperanto, where in the absence of any established tradition everybody simply carries over the syntax of their mother language. Is verda stelo the same thing as stelo verda? Depends...

@AlexP ... are not the place for such a discussion and citations. I'd be happy to analyse the strengths and weaknesses of English from a linguistic point of view as a global language compared to e.g. Spanish or Esperanto, but not here and now (and it has been done ad nausea many times anyway)

"its writing system is a horror descended straight from some Lovecraftian nightmare" - I've learned a bit of Chinese, and I wouldn't say it's quite THAT bad. Tough, but it's way, way, way less scary than I imagined before learning a bit of it (I know about 1000ish words)

@AndrewAlexander: If learning to write 1000 words is considered an accomplishment, then the writing system is a nightmare. In what other language is anybody even counting how many words they can write? Not to mention all the lateral problem it induces, for example, how can one make new words when nobody knows how to read them? No quarks, no enthalpy, no quaternions, no radar, sonar, laser and maser for the Chinese.

It's very agglutinative - they just add a bunch of words together. Think of Chinese characters much like you would root words in a European language and it makes sense. You don't need a new character for each and every concept. For example, when I say 1000 words, I really mean 1000 characters (3000 are required for full literacy, though at 1000 you can understand about 85-90% of what you read)

For example, the word, "last year" would essentially be equivalent to the phrase, "Go Year" in English. 去年. I'm not saying the system is perfect, I'm saying it's not as hellish as it sounds to European language native speaker ears.

@AndrewAlexander ironically, it was Mao Zedong who observed that the written form of chinese language was the biggest thing holding them back.

would add Italy to your examples list - they have many dialects/languages, which they speak so as state-level language Italy, so as Spain which has a lesser number of languages but which also actively are used locally(at least Catalan, as I was informed) and Spanish as primary, spain. So yeah, the problem is not new.

@MolbOrg: That's the case for just about any country. In addition to the their notiional "national" languages, the UK has Welsh and Scots, Germany has Low German, the U.S.A. has Spanish, Navajo and so on, etc. etc. Not to mention countries like Canada, Switzerland or Belgium which openly keep multiple co-equal official languages, showing that in actual practice a single common language is not really needed. Fun factiod: the Roman Empire worked quite well with Latin in the West and Greek in the East, and never tried to force the Greek-speaking Orient to convert to Latin.

To quote someone near and dear to me about Russian from someone who lived in the USFR (now Ukraine), "It was the union language, everyone spoke it." Which is to say the USSR got people on the everyday level to speak reasonably good Russian, and people in cities to prefer Russian without outlawing other languages.

@AlexP countries like canada actually anti-examples, which they can be presented a such. Italy is quite outstanding in terms of dialects which I guess probably not always can be called dialects(maybe it is not entirely right to call them such) - it is more than 40 of them in Italy, and plenty of distinct enough to be not understood by others. If not the standard one - it would be a Babylon, take look at map those are live dialects, they are spoken

@MolbOrg: Italy is home to several Romance languages, of which standard Italian is but one. How many languages is hard to say, due to the general problem of distinguishing a language from a dialect other than by checking whether is has an army and a navy. Generally, at least Piedmontese, Lombard and Sicilian are recognized as distinct languages.

@MichaelShopsin on other side, to achieve this state, the russians had to ban use of Ukrainian in printing, theater and literature for two centuries, then to literally kill the most of Ukrainian educated class and even after that they had to put TV, books and other media under hard control coupled with several decades of propaganda and discrimination of non-russian speakers to make people "to prefer Russian" in everyday life.

@AlexP they hold dear those dialects, use them like family languages and to distinguish who come from where. I was actually surprised by such a linguistic situation there. There is no chance I'll find that now, but one of the dialects I remember to listen was about 70-80% german, not sure if it is one of your mentions and it more likly part of Gallo-Italic languages, so as an example of another group Italo-Dalmatian languages So situation is more interesting than just 4 languages.

@Molborg: There is a beautiful map of the linguistic situation in Italy on History SE.