Hello,
I thnik you can start by reading past discussions about the topic.
There was a GSoC project in 2017 that was not completed (only resulted in a test application showing font rendering with Harfbuzz, but no integration with app_server): Deepanshu's blog | Haiku Project
You could check if that test application still works with current versions of FreeType and Harfbuzz.
A forum thread from another GSoC application in 2024: [GSOC 2024] Complex Font Rendering Update - #4 by PulkoMandy
And another discussion from later in 2024: Right-to-Left (RTL) Languages Support
Various related tickets (some are related, some not really but may be a good place to start to discover app_server code):