Will LG Ultrafine 5k work with 2012 MacBook pro
Firstly, the Ultrafine 5k only has thunderbolt 3 over USB-C inputs, so you'll need a thunderbolt 2 to USB-C cable to even connect. You should be able to run the monitor at a greatly reduced resolution which is then "upscaled" to 5k. For instance, many people run 4K monitors at 1080p, so the 5k should work at 1080p (or other resolutions &lt;5K) as well. 

Upscaling is very common on TVs at the moment, since there is a lack of 4k content, and thus those watching 1080p content on 4k TVs must have their content upscaled. I've attached an article on C|Net discussing upscaling.  

Thus in principle it should work. I'd contact the manufacturer directly to make sure. That monitor is not cheap.