[cpach]

Interesting idea.

I imagine Emacs would be able to support this if you’re willing to dive into Elisp. You would probably need some heuristic to guess language (per line?).

However, have you considered a much easier solution: Use a typeface with good glyph coverage. The DejaVu family is probably a good bet.

[greenn]

As a baseline solution "language" could be substituted for "unicode range".

Re "use a better font": fonts are something I enjoy and I try new ones frequently.