I have auto-generated input with mixed language data (don't ask) that I need to typeset. Thankfully there will be no more than two languages at a time and I will know what the two languages will be, but sadly I will not be able to tag the content with proper markup to delineate the language inside strings. For example I might have a string:
All the documentation I can find for font fallbacks has to do with setting what happens when a font face is missing. For example if I setup a custom font family for a font that doesn't have a bold variant I can set it to use the italic instead, or if it doesn't have a mono-spaced variant in the family one can fallback to a default or other font family entirely.
What I can't figure out is how to fall through from one font to another when the font is missing a glyph. For the example above I am typesetting with the Noto family of fonts and I would like to have Noto Naskh Arabic as the default font, but when it hits Latin characters that don't exist in the Arabic font I would like it to use Noto Sans instead?
How can such a glyph fallback pattern be coded?