Programming with MODx API is not so hard, although the API is poorly documented.
The better manner I found to program with it is to type the API function names in some search engine and see what others have done with it.
Look at the code of some exisiting snippet and try to understand how it works.
Also look at the structures of MODx tables using phpMyAdmin. Table names are quite obvious, as well as fields.
I think that doubling the structure built for a language is not so bad, as you have the same TVs and pagetile, longtitle (, ...) in all languages. When you add a new TV for content you don’t have to create one per language.
It is probably possible to add some buttons to the backend to create the translated docs (if not exist) and switch to them for edition.
An advantage of plugins is that you can associate events with the PHP codes, for example perform some action when the changes in some document are saved.
I think that the best way to do such think would be improving some standard duplication snippet like mine and then transform it into a plugin so that translating becomes easy from the backend.
This is just a question of time and willingness.
I never programmed plugins myself (only snippets), but as you have already some PHP experience, I think that 2 to 5 days programming would suffice you to discover the MODx API and create a very flexible solution.
I read somewhere that MODx revolution is fine once you understood it. And it is multilingual from what I heard. But from my very small experience with it (and it is also the opinion of many MODx users), MODx Revolution’s interface is very deconcerting at the beginning.