Last night, I was reading a thread on reddit titled "When did the use of gender start showing up in language, and what purpose did it serve?". One of the top posts, by a user named ggchappell, said that the original poster seemed to have it the wrong way around: languages "start complicated and sometimes get simpler", especially when those languages are used for trade. In response to this, user mojitz asked,
"The simplest languages are those that ended up being used for trade between multiple groups of people."I thought that was a really exciting question, and was prompted to write the response below.
Does English conform to this generalization by your estimate?
Depending on how you define "simple", English is a relatively simple language, but probably not because of trade.
The question of how to define language complexity (or, in fact, if languages differ in complexity in the first place) has been a hotly contested, very political issue in the world of linguistics. Because of this, it’s really only been in the last few years that we’ve made substantive advances toward finding quantitative metrics of language complexity. Some really exciting work was done a few years ago by Max Bane, who applied Kolmogorov complexity – a concept from information theory that measures how densely you can compress some data without losing any information – to the inflectional and affixal morphology of various languages . This is to say, instead of measuring how complex a language’s sound system is, or how difficult that language is to learn, Bane was looking at how complex the words of a language are in terms of their internal structure: the rules dictating which prefixes and suffixes you can (or must) add to a "base" word form, e.g. "type" becoming "typed" to indicate the past tense.
If we use Bane’s metric for complexity, we end up with languages like Hungarian and Latin (which, as others have pointed out, has a very rich inflectional system) being ranked as most complex, while morphologically "isolating" languages like Vietnamese and Chinese – languages that have on average fewer units of meaning per word (contrast "establish" with "anti-dis-establish-ment-arian-ism") – are ranked as the least complex.
Slightly more complex than isolating languages are a class of languages called "pidgins". Pidgins are communicative systems formed when two or more groups of people without a common language need to communicate. Getting back to what /u/ggchappel said, pidgins are very frequently the result of different language speakers coming together to engage in trade. Moreover, pidgins are very widely held as being particularly simple languages - in pidgins, we very often observe things like the reduplication of words or morphemes to augment their intensity (e.g., in Nigerian Pidgin, "sikisiki", meaning "someone who is always sickly", and "E big well well" to mean "It is very big" ). When children grow up learning a pidgin, it transitions into what linguists call a "creole", and usually as a result of this undergoes an increase in regularization and linguistic complexity. However, children don’t normally grow up learning a pidgin as a result of trade – this is much more likely to be indicative of one of two situations: either multiple groups of people without a common language have been brought together as a result of slavery, or one group of people has invaded another. In the latter scenario, we see the establishment of a "prestige" language – a language of the elite and the learned, the language of the conquerors – and a "vulgar" language, the language of the conquered masses.
Now, this is where things start to get really neat. Let’s take a quick look at the history of England (and English) from the 9th century through to the 13th century. In 9th century England, Old English was being spoken, a Germanic language strongly related to other continental European languages. In northern England, however, English was hardly being spoken at all: the Old Norse-speaking Danish had invaded, and established Danelaw. At this time Old Norse and Old English would have been mutually intelligible, much like modern-day Swedish and Norwegian – a speaker of Old English would have been able to understand a speaker of Old Norse, and vice-versa, although probably with some difficulty. It’s very hard to overstate the amount of influence Old Norse had on Old English, but to give an example, the pronouns "they", "their" and "them" – core components of Modern English – derive directly from the Old Norse "þeir" ("their") . But the amount of change Old English experienced interacting with Old Norse pales in comparison to what happened next.
In 1066, William the Conqueror invaded England and, in doing so, established French as a prestige language. Now suddenly we see incredible amounts of French influence on Old English vocabulary and grammar, so much so that by the 12th or 13th century Old English isn’t even Old English anymore, it’s Middle English. To give you an idea of how radical a change this was, Modern English speakers can understand Middle English, whereas Old English might as well have no relation to what we think of as English today. Middle English is the language of Chaucer, who in the 14th century wrote lines like
"Whan that Aprill, with his shoures soote The droghte of March hath perced to the roote And bathed every veyne in swich licour" .Middle English is the language of Orm, who wrote in the introduction of his 12th century book Orrmulum,
"[Th]iss boc iss nemmned Orrmulum, for[th]i [th]att Orrm itt wrohhte" .Old English is the language Beowulf is written in, which contains such unintelligible passages as
"Hwæt! We Gardena in geardagum [th]eodcyninga [th]rym gefrunon hu [th]a æ[th]elingas elle fremedon" .About 90% of the words in Modern English are from either Latin or Greek, with the vast majority of these having come through French during this time period or later  (although roughly half of those used in everyday speech are still of Germanic origin ).
Let’s get back to Bane. You might have noticed that I didn’t mention where English ranked in Bane’s complexity survey. Let me tell you, English is much less complex than Latin. English is much less complex than Icelandic, which has changed so little as to be mutually intelligible with Old Norse. English is less complex than French or German. In fact, in Bane’s ranking of 20 languages using his complexity metric, English was less complex than any other European language. Instead, English falls somewhere between Dutch and Maori. More notably, though, the English language’s complexity is, at 16.88%, closer to Nigerian Pidgin (at 9.8%) than French (at 23.05%). Old English underwent creolization to become Middle English .
In short, and to answer your question, yes, English can be construed as being one of the least complex European languages, not because of trade – but because of conquest.