You can absolutely share resources between projects! (A fact I discovered far too late on my first project after replicating an image repository with almost 1GB of files 10 times…). I keep mine on a network drive dedicated to storage because I work across multiple VDIs, and I can access/edit images no matter which one I’m logged into.
You can do like you said, put the media in a central location, and when you bring them into Lectora, you only have to specify the location of the media. I’ve found that when starting a new project, Lectora seems to remember the last location I used to load images from.
As for publication, Lectora will include any images that were used in your slides – if you have, say, 100 unique images in your repository and you only use 20 unique images in your project, Lectora will only include those in the published files.
Based on your OP, it sounds like you’ve made 4 distinct projects, one for each language you are looking to support. If that’s the case, you should know it is possible to make a single course that houses your multiple language content, instead of splitting it into distinct project files. That would greatly reduce your project file size as well!