@codinghorror @zogstrip @Mittineague et al - I located the source of the bug!
It’s being caused by the HubSpot tracking tag. Specifically, by their new ‘Lead Flows’ feature which allows you to present fly-in modals with attached forms (for blog subscribes etc.) Despite the fact that the feature is only supposed to be active on our blog page, it appears the update to the JS tracker is hijacking all i18n translations to run through its domain. Leadin.com is the domain used to host assets for this new feature, which is why my discourse issue was looking to that domain for all translation files. Super weird bug, but at least it’s fixed by disabling the HubSpot tag on our forum for now. Lost quite a few brain cells over this one.
Thanks for your help!