Not as good as Google’s thing, it’s a pain to use. “namaste” (नमस्ते) comes out as “naa-maa-saw-tae”(नामासते), and that’s the least inaccurate example

I don’t just want character substitution.

It’s for an Indic language. I want it to be so when I type in “namaste”, it types it out correctly as: नमस्ते I want it to be offline.

(afaik this is readily available on android devices using google’s input software)

privacy-enhancing technologies

Ahh yes, of course!