I followed the discussion about international documents for a while,
then yesterday, reading a German math book, it occurred to me that the
following point (as far as I can remember) has not been discussed:
It seems standard in German mathematical typesetting to use
letterspacing for "emphasis" (e.g. for defining words), while italics
are reserved for theorems and such.
So this concerns only a "sublanguage", rather than default German
typography. On the other hand the problem seems to be general enough
to have a solution within LaTeX.
I am aware that Don Knuth calls letterspacing in TeX a "bad idea" and
would prefer to have fonts with custom made kerns for wider
inter-letter-spaces. However, with the advance of NFSS and the wide
proliferation of fonts from different sources this seems to pose
unnecessary constraints on the user, with at best epsilon improvement
in the output. So I think there is a good case for this to be handled
automatically by TeX/LaTeX.
The currently available letterspace package does not seem adequate
because of its non-LaTeX syntax and the fact that it breaks the
kerning between pairs of letters, which does look bad especially for
small spacing factors. I have a modified version of letterspace due
to Donald Arsenau which solves the latter problem, and proves that
this can be done without modifying TeX (although I guess modifying TeX
could provide a more stable solution).