How to implement side-by-side translation? How can you set up a translation system in Drupal which displays, for example, the English and the French node edit forms side-by-side? Is this possible using any of the contrib modules?

Additionally, how would I go about setting a requirement that a translation must always be entered when entering English content. Eg, to publish a piece of content, it must have both an English and French translation. Maybe through rules?
There hasn't been a whole lot of public work on this that I can find.

Some module options:


Side-by-Side Translation (no code)
Entity Translation Side-by-side Editor (code, no release)
Dual-Language Editor (code, no release) 


liquidcms helped me find that last one.

I'd recommend trying the latter two and seeing how far you get.  Post patches for missing functionality as necessary.

Also, see the issue Create Node With Multiple Translations Using Single Form.