This is not a common thing by any means, but I do suspect that it happens because of a particular bug in CX.

Examine this revision created by CX: https://he.wikipedia.org/w/index.php?title=%D7%99%D7%97%D7%A1%D7%99_%D7%99%D7%95%D7%95%D7%9F%E2%80%93%D7%9E%D7%A7%D7%A1%D7%99%D7%A7%D7%95&action=edit&oldid=27229517

Search the wikitext for en→he

This blob was not supposed to appear there. The source wikitext is just a link:
* Foreign Minister [[Bernardo Sepúlveda Amor]] (1986)

The blob includes a reference to the file Bernardo_Sep%C3%BAlveda_Amor_-_Flickr_image_5938295386.jpg . This file doesn't appear in the translation source article, but it does appear in the target article for the link, Bernardo Sepúlveda Amor, so perhaps it leaks from something that previews links: the Previews extension, CX's own link preview, or something else.

@Nikerabbit found some code that may create wikitext like en→he, but it's not supposed to run in production:
https://gerrit.wikimedia.org/g/mediawiki/services/cxserver/+/ad369868dde65aaf76f9fdf9743e2b451f090a15/lib/mt/TestClient.js#37