"Example: get the content of a Main Page." links to https://www.mediawiki.org/wiki/API#A_simple_example, which no longer exists.
We should either delete that entire phrase, or maybe link to a https://www.mediawiki.org/wiki/Special:MyLanguage/API:Tutorial instead.
The message key is apisandbox-intro and it's in the mediawiki repository (core).
You only need to change en.json and qqq.json as all other messages will be updated by translators at translatewiki.net