Nice initiative Tom, we are also interested in sharing concerns and good practices in this topic.
To talk also a bit about our project, we are transforming the visual archives of the French Institute for Anatolian Studies: http://ifeaistanbul.net/photodatabase/, in particular turning from Omeka 2 to Omeka S.
Our requirements are that the website would be available in English, French and Turkish (navigation, description of images and tag cloud) with the ability for the user to switch languages at any time, within the next four months (July 2017).
We now store our items once with three times duplicated fields imported with CSV files, as for now we are manually setting the language but we welcome the future possibility to do the mapping with CSV Import with great interest.
We are ready to help for the Transifex translation of Omeka S, especially as French or Turkish might not be among the first languages that will come up. Do you advise that we open a project now?
As for the display of only one language by site and the possibility to switch between languages, we are prospecting and not set yet on something, Patrick suggests a theme, Tom you talk about a module, do you know what has been adopted for this website of the National Library of Wales: http://cymru1914.org/? Have you heard of other examples of multilingual Omeka S websites?
Thank you in advance for your insights and good luck with your projects as well.