I invite these optimists to try travelling from York to London in the UK and ordering a Maccy D's. Come to that, try asking if you can wear thongs into a restaurant in Newcastle and in Sydney. I guarantee two very different responses, but I'd only bother standing by with a camera for one of them.
This is because English has several oddities out of the gate. It's spoken widely, and suffers from all the inconsistencies normal to wide geographical spread. Put a Scouser and a Texan together at an open bar without an internet translator and watch the fun.
English is built from a smattering of Celtic overlaid forcibly by Latin, in turn overlaid by Saxon and then Norman French, meaning it takes part of its vocabulary from the largely Germanic North, and a lot of it from the Romance languages to the South.
Have a look at some untweaked Chaucer (c. 1343 – 1400), the first person to actually write in English, rather than French or Latin. At that point in time, actually writing literature in English would be roughly comparable to someone now writing a work of philosophy in text-message shorthand ... doable, but something of a freak of nature.
(From the Knight's Tale)
Whilom, as olde stories tellen us,
Ther was a duc that highte Theseus;
Of Atthenes he was lord and governour,
And in his tyme swich a conquerour,
That gretter was ther noon under the sonne.
Ful many a riche contree hadde he wonne,
What with his wysdom and his chivalrie;
He conquered al the regne of Femenye,
That whilom was ycleped Scithia,
And weddede the queene Ypolita,
And broghte hir hoom with hym in his contree,
With muchel glorie and greet solempnytee,
And eek hir yonge suster Emelye.
Certainly you can unfocus your brain a little, and stare at it, and you can make it out without too much trouble. If you have some grounding in Classic Mythology, you can cheat and fill in any blanks (cheat. There's another good word, etymologically. "To escheat," a shortening of Old French 'escheat'.) If you feel particularly technical, you can even take a red and a green pen and highlight the words that are clearly Saxon in origin, and which are clearly Latin / French.
Let's take one of the words there (just one, in the interest of having a post that doesn't equal a full-length novel). Let's look at contree. Obviously, modern English, 'country'. Or you could say 'nation'. Starting to feel a twinge of sympathy for those who have to actually learn this language the hard way? I do. You've already got two completely different words meaning pretty much exactly the same thing.
Among other oddities from its mixed heritage, that means that English has nearly twice the vocabulary of most languages, most estimates pinning it at somewhere near 1 million. Given that the majority of English speakers actually commonly use about 5,000 words of that, and even a highly-educated university graduate only about 20,000 ... that's still a lot of variation for a second-language speaker to try to master.
For example: fish is a plural noun. Except when it's not. Anyone want to take a stab at why and when you can actually say fishes? What's the difference between a belfry and a belltower? Come to that, which English-speaking populations can you insult by calling them a bellend, and which will just look at you blankly?
As practical choices go, picking English as the 'universal language' scores a resounding E for effort. It's hard to pronounce, regional dialects vary wildly, and the vocabulary is, if possible, more enormous even than the number of grammatical irregularities.
Of course, as far as writers go, that makes English a whole field of fun with occasional streaks of psycho. You can do nearly anything in English. (Well, you all knew I was going to end up talking about writing.) You can turn up ten or so synonyms for pretty much any word you care to use (or not use. That's what synonyms are for.) And if you care to dig yourself into regional slang for some character colour ... well, the Urban Dictionary is a writer's boon there. If you weren't planning on slang to start with, you'll almost certainly end up wanting some after ten minutes in there. Not to mention if you put any two grammar nerds into a bar with a pitcher of beer, you can get five different opinions on something as basic as when and where to put a comma (Oxford commas, anyone?).
Basically, English as a universal language is a moderately shitty choice. Why do I blaspheme? Well, because language, at its most basic, and basic is really what you want as a universal interface, is a means to communicate easily and clearly. Yup, really. I can lose most native speakers in three sentences if I make the effort. English has weird pronunciation, which varies wildly depending on region. It has a massively complicated grammar structure. And don't forget that huge, doubled vocabulary. As far as simple, clear, universal communication goes ... well, some of the Eastern writing systems might, possibly, throw more of a wrench in the works, but only by a whisker.