I sincerely believe that becoming perfect in a language requires a lot of daily hard work and actually living in a respective country. In fact, seeing my parents misusing the Portuguese language still with some very obvious bad habits, even after they lived and worked there for more than 12 years, almost only talking Portuguese, I think after a certain age it becomes incredibly difficult to acquire a foreign language to such a degree that you become indiscernible from a native speaker, at least for Average Joe.
My above post though... Very bad usage for which I can only blame myself and the early morning. And learning proper English on the Internet is close to impossible. How most Americans use English has become a gobbled mess to be honest (e.g. "irregardles", or "that was uncult for").
I've seen this with Spanish-speaking people. My wife was born and college educated in Mexico, has a Masters degree from here in the US as well. I've gleamed some insight from her. Many immigrants to this country, from all nations, don't know their native language. They can survive wherever they're from, but they don't really know Spanish or Cantonese etc, they'll use terms commonly used by the uneducated like "patas" for their feet (patas = paws in Spanish).
I can attest that there's no shortage of monolingual English-speakers who are no different, and don't really know their own language. They can survive here, but are barely educated enough to survive in the US. Some can't, and rely entirely on social programs to make it through life, a large part of the reason is general lack of education. Many Americans are entirely unemployable, and many are already "over-employed" and lucky to have jobs at all. You could easily be meeting up with those people online.
First of all, Spanish has a whole other problem, because there is no "official" Spanish AFAIK (even though people from Madrid claim that), compared to e.g. German and "Hochdeutsch". If you travel around Spain, just half a day from Madrid, even well educated, employable people will use words and constellations nobody would use in Madrid. And I think that is totally fine, people adapt to it. Considering the whole Latin American region and their variants, Spanish is a mess in this regard (they officially write words differently, using 'x' instead of 'j' in several occasions), but you can tell where somebody is from.
Regarding the deterioration of English: perfectly employable, smart, educated people in high paying jobs use "irregardless" (that missing 's' was my mistake), and it somehow made it into dictionaries, and it seems you think it is a word too. But think about it, does it make sense?
Why would you negate regardless and what does that construction mean?
Reminds me of when an Indian friend in high school played a prank on another friend by impersonating me through instant messages (we were sharing a hotel). My friend was able to figure out that it wasn't me because my Indian friend used the phrase "today morning" instead of "this morning."
Back then, we didn't have whatsapp, so we were texting. She never responded to texts during lecture before that day, so I was shocked.
Then, one message later, I realized that it was someone else using her phone and fifteen minutes later, I said, "If you are done, please give her phone back to XYZ"
He'd say "I order the fries" to the waiter.
Likewise I can tell that English is not your mother tongue.
I understood everything you wrote (you're obviously fluent), but tiny little things give away that English isn't your native language:
> learner's with native tongue that hasn't these features
I would have written: learners whose native tongue doesn't have these features
> but we just let it slip
but we just let it slide
> you've not grown up here
you didn't grow up here
I sincerely hope you don't take my comment the wrong way. It was too interesting to pass up an example of the very thing you mentioned: that you can tell even when the person is fluent (even if you don't hear an accent).
Related observation: You know how Ted Kaczynski, the Unabomber[1], was caught? He used the expression "You can't eat your cake and have it, too."[2] But nearly everyone knows the expression as "You can't have your cake and eat it, too." Ted Kaczynski's brother recognized Ted's highly unusual usage from the anonymous manifesto published by the Unabomber, and this is what convinced him that he was the Unabomber.
[1] https://en.wikipedia.org/wiki/Ted_Kaczynski
[2] http://itre.cis.upenn.edu/~myl/languagelog/archives/002762.h...