The presence of active translation extension tags on a page suppresses resource loader. Affecting visual editor, wikitext editor toolbar, central notice etc.
Example:
Suppressed (SubPage): https://meta.wikimedia.org/w/index.php?title=WikiWomen%27s_Collaborative/Twitter/ast&banner=WLM17_gb_test0&uselang=en&force=1
Suppressed (Main Translated Source Page): https://meta.wikimedia.org/w/index.php?title=WikiWomen%27s_Collaborative/Twitter&banner=WLM17_gb_test0&uselang=en&force=1
Working (No translate tags present): https://meta.wikimedia.org/w/index.php?title=Steward_requests/Username_changes/2011-11&banner=WLM17_gb_test0&uselang=en&force=1
Translatable pages work fine on other wikis (tested on mediawiki.org and Commons).
The bug cannot be reproduced when appending &debug=true.