Who writes and translates the example sentences?

A lot of people.

Writers are always Japanese native speakers.

Translators consist of native speakers of both English and Japanese.

Typically speaking, the flow works like this:

A native speaker of Japanese writes an example sentence
Another native speaker edits it for quality and naturalness
Voice actors record natural-speed audio for the sentence
A lead editor who is a native Japanese speaker approves the sentence and its audio
A native speaker of Japanese translates the sentence
A native speaker of English checks the translation for accuracy and naturalness
A lead editor who is a native speaker of Japanese approves the translation
A lead editor who is a native speaker of English approves the translation

This is done for every single sentence you see on NativShark.