Okay, I somehow got it to work. After adding my custom field to the Purchase Orders page using Page Designer (in the global context) and publishing it, I went to “Translatable Texts” page as suggested by the Cloud documentation: https://docs.ifs.com/techdocs/22r1/040_tailoring/225_configuration/200_client_configurations/350_translations_in_configurations/
There I searched for my specific field ([global].PurchaseOrders.list…..) and added my Finnish translation (NOTE: These seem to have nothing to do with the translations set in the Entity Configuration page? I guess those are just a useless relic from the old EE client design...).
After that, I refreshed the language cache and refreshed the Purchase Orders page in Chrome. It still didn’t affect my field, but then I tried once more to republish the page configuration in Page Designer, and then the correct Finnish translation appeared.
Not the most straight forward flow I would say, but atleast it’s working now...