(Side note: those are not a and o with diacretics, they are entirely separate letters in the alphabets of the Nordic countries, with entirely different sounds.)
Side note: helping my French housemate with his uni assignments was an experience, none of the symbols were where I expected them to be
I've used US keyboard layout since I started programming (my first mentor essentially forced me to switch to it, he was right about it being easier), but throughout the years been using Swedish, Norwegian, British, Spanish and French physical keyboards, never cleanly mapped to the actual layout I've used on the OS, and never been an issue.
The last part though, is a real one, trying to pair program with Spanish programmers always have at least one moment of holding Shift and sliding the finger across all numbers to see where that specific symbol actually is.
If it's a key that you may not often type and one that is often transposed between regions, the fact that the entered char is not shown can lead to frustration.
e.g. " and @ are in different positions in UK vs. US keyboards. So user thinks they are typing @, but " goes into the box.