Who writes and translates the example sentences?

A lot of people.

Typically speaking, the flow works like this:

1. A native speaker of the target language writes an example sentence
2. Another native speaker edits it for quality and naturalness
3. Voice actors record natural-speed audio for the sentence
4. A lead editor who is a native speaker approves the sentence and its audio
5. A native speaker of the target language translates the sentence
6. A native speaker of the source language (the language the sentence is being translated into) checks the translation for accuracy and naturalness
7. A lead editor who is a native speaker of the target language approves the translation
8. A lead editor who is a native speaker of the source language approves the translation

This is done for every single sentence you see in the platform.