While I don't speak German, I speak Korean, and can comment in general on the issue.
The article does a decent job of outlining some of the issues.
At the core, you have different consonants and vowels in different languages, and even in different consonants and vowels inside of a single language (dialect or accent).
For English speakers, try saying these 3 words:
1) Mary (the name)
2) Merry (as in merry Xmas)
3) Marry (as in get hitched)
The a/e there will sound the same to some, while sounding different to others, depending mostly on where you are from. You may hear 1, 2, or even 3 distinct vowels.
As another typical example, Americans typically pick out Canadians by the way they say "about".
Next, you have different combinations of those vowels and consonants.
And that pretty much sums it up. Different combinations of vowels and consonants condition you with motor skills (very much like muscular memory, if not exactly that) so that when you go to step "outside" of those "rules", things can tend to get weird.
Again, for English speakers, try to pronounce this as a word:
If you can at all, chances are that you slaughtered it because the initial "ng" sound doesn't exist in English. Again, while that is a real word, it is used in other words as well, e.g.:
Which again, is very difficult for English speakers to pronounce.
There are tonnes of examples out there of sound patterns that don't exist in different languages, so "squirrel" in the mouth of a German speaker isn't very surprising. Though it may be entertaining!