As a ground rule we mostly wish to be understood clearly so don't write anything that might be misunderstood in context. Thus "harbour" or "harbor" doesn't matter as both are understood whichever side of the Atlantic you come from. However, "subway", "vest", "4/5/2012", "3.08" (feet/inches/dollars/pounds?? are fraught.
Sometimes of course we wish to be vague, or smokescreen the truth, or to one-up, or break the norms, or inter alia sound like an academic prat. Or indeed sound like Chaucer.
Each to their own!

@Jo Yes I think you are taking a true and fair view of the situation; and I think auditors in the almighty US of A use similar wording. A green tick to you!

@Warsaw Will As I said before, it's not the words themselves but the way some people use them; and often it's the latinate words in English which are used in a sort of linguistic one-upmanship game especially in academic circles.
I too taught in Central Europe for several years (not in Poland itself though) and it certainly seems like a boon and so easy to deal with latinate and french words that are in English too.
However for the past ten years I taught many Chinese, Japanese, Korean, Fiiipino, Indonesian, Hindi, Arabic and Thai speakers too; and when one gets to IELTS and academic English, all those latin-based words become a pain - moreso than phrasal verbs, although the latter are far more use in everyday speech. Words like "rural" - why on earth did this get borrowed, when "country" would be good enough. (Sometimes I feel like I'm teaching latin and greek! especially when explaining prefixes like con- syn-). So much of what is called the "formal" register is just putting in a latin-rooted word where a saxon one would do - "decapitated" instead of "behead" .
Korean does in fact have more than a few hundred words borrowed from , well, American I think. Alas the pron and meaning often somewhat changed too!

I do agree however that some latin-rooted words do add an other nuance.

You are quite right about Celtic tongues - perhaps one day there will be a revival in the west of England, everywhere west of Reading!

"The Latin and French basis of many of our words makes it considerably easier for us to learn other European languages, and for speakers of European languages to learn English."
And the germanic-rooted words make it easier for central Europeans to learn English!
The latin and french ones only help romance-language speakers.
Perhaps we should write English with chinese characters: that would make learning Chinese, Japanese and Korean easier!

I am with Holy Mackerel on this: latin-rooted words often sound effete, elevated, and overly academic. Take another butcher's.

If one looks at ESOL textbooks like Headway, they often use "lead-in" for "introduction". Why ? Because it's easier to understand?

For instance: I have heard an American say "Did you do your homework already?"
and "Did you do your homework yet?". Both to me are quite awkward-sounding, but seemingly okay to normal to some, and readily understood.
The other issue that arises is register and nuance. For instance "pussy" was not regarded as a risque word in the UK until perhaps recently under American inflows.
So sentences like "John I love", which are slightly unusual or poetic although natural in context, are still part of "standard" English; even archaic English (Tolkien, KJV) is still standard as, despite having an "archaic" flavour is would still be perfectly well understood by an educated English speaker worldwide. If one couldn't reasonably put it in a proficiency exam for English, it may be non-standard.
"As of", "as at", "as from" are all to me normal business-speak like "with effect from".

The difficulty with "what sounds natural" as a criterion is what sounds natural to you may not sound natural to me, or someone from say Jamaica or some other dialect. I don't know what a better criterion would be: perhaps there is more mileage in the idea of "standard" English - as being anything that a well educated English speaker would understand clearly and could use - and non-standard English, stuff that is strictly dialectic or obsolete. Quite who decided which is which I dunno, but it needs to be empirically based on corpuses.
Let me add that I am not suggesting that dialect words are in any way lesser or nether; just that one could not be sure of being understood forthwith in an international email or conversation.

" Pashto is a second-class language "
At the end of the day it is not the source of the word that matters; it is people's attitude to it. Some borrowed words enrich the language - "kindergarten" must have been brought in to fill a gap. It really makes no sense to say that "kindergarten" is okay but "au pair" is not - but "kinderschwester" would be. No, upper class English people have a "nanny", middle class, an "au pair", and lower class a babysitter if they can afford it. Rich people used to employ a nursemaid. Perhaps if we could all afford a nanny there would be no distinction. At root it is snobby attitudes not word origins that count.

"it's -11 C outside!" I wouldn't have missed my time in Eastern Europe not for all the tea in China. Boots and fur-lined leather coat. Just memories.

@WW you're quite right - "don't have to" vs "must not" is vital.
I was thinking more of how some of the old (Headway?) books used to harp on about the between "I have to go" and "I must go".....
Yes if there is an L1 false friend one would of course have to deal with it - horses for courses.
"I presume I'm the one who's "harping on" .. " - no , nie jest obraził (obrażony)

Even in internal company emails it pays to err on the formal side - esp if emailing the boss. Formal English is the real struggle.

Re teaching English as a "second" language:
Today the need often for business or academic English - emails and essays - and some of the course books are beginning to show this. This means that much less weight may be given to "I've got (to)" - "oh many Brits use this instead of 'i have' " ... and move on quickly instead of making a huge fuss about it like before.
In the same way harping on about the nuances between "must" and "have to" is fruitless - there are far more useful things to be aware of; a wide word-stock is wont to make for better understanding on both sides in real life, IMHO of course.