Unfortunately, your complex script shaping for Arabic and Devanagari is wrong. The Arabic is missing the joining (all forms are isolated), and the Devanagari doesn't have the vowels combining (so you see those dotted circles).
To fix this you'll need Harfbuzz or something similar. Taking a quick look at the code, it seems like you're just doing a glyph at a time through the cmap. That, uh, won't do.
To fix this you'll need Harfbuzz or something similar. Taking a quick look at the code, it seems like you're just doing a glyph at a time through the cmap. That, uh, won't do.