The locales feature has always been designed for theme UI to switch to another language. I think you’ll have a hard time not creating custom templates for different languages. For example some languages have different writing directions and extended characters that might require different fonts. Just something to consider when trying to approach the development of a multi-language site.
You’re of course welcome to make a contribution to the official Ghost open source project, as long as you follow the guidelines provided here. However I will say that despite this being a feature that’s been requested before, it’s not something that’s be considerably high on the list of needed features in Ghost. Building a site to cater for multiple languages is hard, regardless of the CMS and tools you use. I would take some time to think about the site you’re trying to build and consider what tools on the market will help you achieve that. If multi-language is high on your list maybe you should look into a tool that also has multi-language high on their feature list.
I’m not trying to deter you from using Ghost, I just wanted to help you pick the right tools for the job 