Steps to replicate the issue (include links if applicable):
What happens?:
The content of the page is English, but it's displayed with RTL direction.
What should have happened instead?:
Since the content of the page is English, it must be displayed with LTR direction.
AFAIK, the content language of such pages is determined by the sourceLanguage key of the JSON content, and if it's absent, then the site's content language is assumed (but @abi_ can correct me if I'm wrong).
Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):
Currently deployed on Meta.
Other information (browser name/version, screenshots, etc.):
I am testing the message bundles feature of the Translate extension, so I'm only tagging RTL and MediaWiki-extensions-Translate. It's quite possible that other kinds of JSON pages are affected by this issue, too, but I haven't tested them.