Apple - How do I use Messages with Google Hangouts on MacOS Mojave?

You can't. Apple removed the option to log in to Jabber with 10.14, Messages is iMessage-only now.

I have attempted to move all the frameworks over from High Sierra. I successfully got the messages application from High Sierra to open on Mojave and it was able to send and receive messages. But I was unable to add a Jabber account due to an error connecting. I think this might require another framework or something else, but it probably works with the System Preferences account setup plane so changing it may cause other problems. I will look more into.

So as the others answers says I don't think its possible.

For those wondering I copied over the following Frameworks

• MessakesKit.franework
• PhotoLibrary.framework
• SocialUI.framework
• SocialAppsCore.framework
• Ofc the High Sierra Messages.Application

I see this has been already answered, but I'd like to chip in and recommend an alternative messenger: https://adium.im/

Adium is a free and open source multiprotocol messenger for all operating systems(including Mac OS).