What does HackerNews think of WP-API?
The WP REST API has been merged into WordPress core. Please do not create issues or send pull requests. Submit support requests to the forums or patches to Trac (see README below for links).
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For comparison, the WordPress REST API was about 4k commits over ~4 years or so (prior to us merging it into WordPress itself): https://github.com/WP-API/WP-API
(That said, commit count is a pretty useless metric.)
You have both https://developer.wordpress.com/docs/api/ and https://github.com/WP-API/WP-API that can be used for something like that.
For button labels, help texts, in-page instructions: use an i18n framework.
For longer texts, docs, etc: throw Wordpress (or some other CMS) in front of the damn thing. You can route whatever needs CMS functionality, say /docs, to WP, the rest to your app. If you want tight integration you can just pull the content from the WP database. If you want to get fancy, you can even install something like WP-API (https://github.com/WP-API/WP-API).