Fallback fonts in LaTeX export for non latin scripts

[Juan Manuel Macías]

The Unicode TeX engines, LuaTeX and XeTeX, have certain features to
apply fonts to scripts (Greek, Cyrillic, Arabic, etc.), without the need
to switch fonts explicitly. But LaTeX does not include any functionality
for loading 'fallback fonts' out of the box. Seeing things from TeX and
LaTeX this is understandable: since LaTeX is a typographic tool, the
user has the responsibility of choosing the fonts and knowing which
fonts to use. But from the Org side things may look different, as the
average user (who may not be interested in typographical or font
complexities) is looking for immediate readability of their texts when
exporting to any format. We know that, when exporting to LaTeX, this
does not always happen, if texts include non-Latin scripts.

These days I'm working on some experimental code to try to provide Org
with some sort of fallbacks fonts on LaTeX export. The functionality
would (for now) be linked to LuaTeX + babel package, since XeTeX,
although it has the ucharclasses package, is more limited.

The idea is to start from a defcustom that is an alist where each element
has the structure (script font). There would also be a default script +
font, for example ("latin" "Linux Libertine"). At the moment it would
only work for the default roman font, but it can be extended to default
sans serif, mono, etc.

The functionality would not be activated by default. When activated, it
also enables LuaTeX as the default LaTeX engine, and on each export a
list of non-latin scripts in the buffer is extracted. Perhaps with
some code like this, which checks for any non-latin characters:

(let ((scripts))
  (save-excursion
    (goto-char (point-min))
    (while
        (re-search-forward "\\([^\u0000-\u007F\u0080-\u00FF\u0100-\u017F]\\)" 
nil t)
      (let ((script (aref char-script-table
                          (string-to-char (match-string 1)))))
        (add-to-list 'scripts script)
        (setq script-list scripts))))
  script-list)

?

Once the list has been extracted, an ad hoc preamble would be formatted
assigning each script the chosen font.

WDYT? Do you think this would be a viable path? I think that in a few
days I can offer something usable for discussion.

Best regards,

Juan Manuel

--
Juan Manuel Macías