WordPress 5.0 has added a new system, based on JSON files, to translate the text strings which appear in the new WordPress editor (Gutenberg). Although it applies to Genesis Framework (a theme), you can see an thorough explanation here: https://studiopress.community/topic/new-genesis-sidebar-in-wordpress-5-x-editor-is-not-translated/. The situation is a bit different from the one you describe in your post, but the basis of the problem is the same.
Even though you fully translate the .PO file and upload the translation files to WordPress, most of the plugin remains untranslated, because of the lack of the JSON files.
Trying to find a solution, I’ve generated the JSON files from the atomic-blocks-es_ES.po file, as it is explained in the link above (the wp-cli command generates more than 60 different files), and then I have uploaded them to WordPress, but it doesn’t work.
This new WordPress translation system is not easy to understand, and I’m afraid that many plugin developers are not applying it in their code. In my humble opinion, the problem is very serious, because there is a growing number of plugins which interact with new WordPress editor, and also a growing number of text chains which remain untranslated because of the lack of the appropiate JSON files. The consequence of this situation is very ugly: after installing two or three plugins which interact with WordPress editor, you get an unpredictable mixture of spanish and english.
By the way, David, if you feel more comforable talking about this issue in spanish, you can send me a message through the contact form of my web site: https://www.labitacoradeltigre.com/formulario-de-contacto/. I will answer by email.
