(Background: I'm an American living in Switzerland, and studied linguistics alongside IT at university.)
I agree, English 'adopts' words from lots of languages. This can make things more confusing for non-English speakers because the meanings may also have changed, and as you say, it is also a dynamic not a static system.
Every language borrows from other languages. For example, look at how upset the French and German speaking worlds are that English words keep infiltrating their vocabulary! What's weird about it is that is that they blame us English speakers for it, even though they're the ones voluntarily using our words! We didn't ask, never mind force, them to use them!
I suppose everybody thinks that the 'version' of English they were taught is the 'true' one and measure everybody else against this 'truth'!
It used to bother me that Americans had corrupted the English language but as I get older, I can appreciate the logic of many of the changes they have made (still rubs me the wrong way though ).
With regards to spoken language, which drives language change vastly more than written language:
The problem is that everyone assumes their dialect/accent is the one that remained true, while the others became corrupted. But in fact, any time a language divides (whether through geographical or social separation),
every branch keeps evolving away from the common ancestor. No descendant is any more "correct" than the other, because the original common ancestral form is always lost (except in artificially maintained dialects, like liturgical language). So when American English branched off (which is actually hard to pinpoint, since there were centuries of ongoing immigration from different parts of the British isles, each wave bringing a different dialect from a different point in its own evolution), it evolved away from the original British —
but so did British English! In fact, among scholars of English linguistics, it's widely accepted that as of now, American English has actually retained
more historical forms than British English!
Another thing to bear in mind is that a lot of modern American pronunciation — such as the heavy rhotic "r" sound — is borrowed directly from Irish English, particularly Dubliner, thanks to the huge numbers of Irish immigrants in the 19th century. To this day, many Irishmen sound extremely similar to Americans. (To the point that they are often assumed to be American when outside Ireland, according to such an Irish friend of mine.)
I've always considered American English as "lazy". Not saying American's are lazy but their language and spelling is very phonetic. AU/UK English can be confusing at first, but once the rules are known, it actually makes sense... most of the time. I also happen to speak Finnish and English is a hell of a lot harder ;-)
But it's not lazy. English spelling and pronunciation were both in flux when they split apart, and that flux has never stopped.
But again, it's often American English that retains the historical form. For example, the British often accuse Americans of being lazy for not pronouncing differently the "a" in "can"("can") and "can't" (Am: "can", Br. "cahnt"). But in fact, British English originally used the same "a" in both, and only began distinguishing the vowel long after American English separated from it.
One final thing about spoken language: One thing that bugs me, as a linguist, is that people compare their country's "best" English (like RP in England) to another country's most distinctive dialects (like hillbilly American English, or Appalachian vernacular, as linguists call it, or urban black English, African American Vernacular English to scholars). You rarely hear the English comparing their strong regional dialects to standard American. (Perhaps because there's a huge cultural asymmetry: the British openly look down on Americans, whereas Americans consider anything British to be noble and upscale.) In linguistics, it being a science, we seek to describe and study language as it is used, not to pass judgement or prescribe how it should be used. But that viewpoint is, of course, irrelevant in the real world.
As for spelling, English orthography was completely unstandardized on both sides of the Atlantic until comparatively recently. Remember, the British began colonizing North America not long after William Shakespeare's death. Shakespearean spelling is no closer to modern British than it is to American, and far more different than modern British and American spellings are from each other!!
A major reason why English spelling is unusually complex is that English rarely re-spells words when the pronunciation changes. English has had several major "vowel shifts" (with many regional accents emerging precisely because they didn't adopt particular shifts, or had minor ones of their own) where pronunciation changed across the board, along with many slow changes in consonant pronunciation. Any time a new word was coined (regardless of how), its spelling reflected the pronunciation of the time when it was coined, but then it remains more or less etched in stone. And that's how we end up with the same vowel having 5 different pronunciations or more! (In contrast, many languages have official standards bodies which routinely update the official spellings and grammar.)
What I will absolutely grant you, and I say this as a well educated American with years of professional writing experience, is that Americans are getting far inferior language education in school. Combined with Americans' overall distrust of expertise (they confuse it with aristocracy!), and aspiration to
mediocrity, it's resulted in a population that rarely uses English in a masterful way.
Although I beg to ask, why the hell don't Americans adopt the metric system?!? It makes so much more mathematical sense.
In many areas, we have. Soda comes in 2 liter bottles (food products have been labeled in both imperial and metric for decades, and nominal package sizes have been little by little switching to metric). Photographic and movie film is measured in millimeters. The American auto industry is mostly finished switching to metric. Pharmaceuticals have been metric for ages. Science is nearly exclusively conducted in metric.
Where the US has remained stubborn in everyday use is in distances (the 1970s effort to switch highway signage to metric was aborted after just a few years, though our speedometers retain both miles and km scales), temperatures (I have to say I appreciate the somewhat higher resolution of Fahrenheit, when it comes to everyday things like weather or body temperature, where having 80% more degrees in the same number of digits is handy. The 32? zero offset is maddening, though!), weights, and to a lesser extent volume.
And of course, I always love to point out to people — often to resulting indignation — that there's no country in the world that is 100% metric, either! For example, small pipe fittings (like shower and hose connections) around the world are nearly universally in inches. Automobile tire ("tyre" for some
) sizes are a bizarre mix of both imperial and metric (210/75R15, for example, means the tread is 210mm wide, the sidewall is 75% of 210mm, so 157.5mm, on a 15 inch rim). We're all familiar with 0.1" lead spacing on DIPs, even if that's going away little by little. And whenever we buy a printer, its resolution is nominally specified and engineered in DPI (dots per inch)*. Similarly, display and TV sizes are always advertised in inches, other than jurisdictions that have outlawed that (but the nominal "size class" is still always defined in inches, as evidenced in the model numbers.)
*Indeed, one of the few devices I've ever seen with a nominally metric resolution is Wacom graphics tablets, which are clearly nominally engineered in lines per mm. For example, their current top models are advertised as 5080 lines per inch, which is precisely 2000 lines per mm. The other is some imagesetters for making offset lithography plates.