For main tutorial: https://ferdowsur.gitbook.io/tinygiantstudios/#creating-fonts
Select the characters your language has while creating font. To figure out which characters your language needs, a simple google search like "Roman Unicode range" would give proper results.
The original font must contain the characters for the 3D version to include it.
If you need characters in range of Few Thousands , You can split it up to several fonts and set them as fallback for a single main font. Not an issue after version 2.0.0