highcoder hi, welcome to Flarum !
We don't have much documentation on that indeed. I believe there were a few topics on this forum to explain the process.
The best way to understand is to have a look at existing translations. https://github.com/flarum/flarum-ext-english is the base english extension and https://github.com/milescellar/flarum-ext-french is a French translation extension for example.
Translations are Flarum extensions, so the developer guide in the dev section will help you get setup if you want to test your translations on a local Flarum install ?
Let us know if you encounter difficulties !