Even with monospace fonts you can't just rely on number of characters once you are beyond ASCII. Han characters are often double wide.
This kind of formatting can't be made to work in the general case, so we're left with what might be least surprising. Where almost any choice is going to be surprising to someone. Given that, I would prefer that it be stable, and therefore independent of locale. 

On Tue, Sep 10, 2019 at 10:36 AM Niall Douglas <s_sourceforge@nedprod.com> wrote:

> Perhaps it would be helpful to enumerate what we expect to be portable
> uses of field widths.  My personal take is that they are useful to
> specify widths for fields where the content is restricted to members of
> the basic source character set where we already have a guarantee that
> each character can be represented with one code unit.

Most programmers would use field widths for padding items so they appear
in a grid. They would expect that 𐐗 padded to eight characters yields
seven spaces and 𐐗, not four spaces and 𐐗 (because 𐐗 consumes four
bytes of UTF-8).

That said, as we have no idea how unicode would get rendered (0, 1, or 4
characters for 𐐗 being the most likely), I cannot improve on your
proposal. The situation sucks, quite frankly.

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