What should I do about HTML messages? I could possibly use some funky regex and extract the text into the modal, then translate it in there as plain text. Or maybe I could try to reinsert the translations back into the HTML structure and try to display that (either in the modal, or in the original ThreadEnvelope). Or I could just not give the option for them to be translated (what I currently set).

If it doesn't exist already we can add an API endpoint that gives access to a text-only version of an email. AFAIK that is also the input for thread summaries, so we have that, but possibly not exposed to the frontend.

@GVodyanov we should also cover this new feature in the user manual and add a note that it's only available for instances that have translation apps/services. In the admin manual we can mention it too with a pointer to the translation services admin documentation (I hope it exists).