Following this post, support for PostScript/EPS output of Greek letters (and any other script)
has been added to FLTK 1.4.
This requires to activate Pango support at FLTK build time, which is done either by
./configure --enable-pango
or
cmake -D OPTION_USE_PANGO=1
With that support, non-Latin scripts can be drawn by fl_draw() whatever is the current font.
That is, it's no longer necessary to match the chosen font with the character repertoire in the drawn text.