Drupal 7 or 8 for bilingual site
Drupal 8 is great, and has a lot of improvements related to multi language sites. But Drupal 8 isn't ready yet, and it "will be ready when it's ready" (that's what some experienced Drupal-ers say, and I do fully understand that).

Also, after Drupal 8 is released (I've heard dates like somewhere in 2016 only, and it wouldn't surprise me), there are still all the contributed modules that will not be upgraded yet to Drupal 8. Today there are still Drupal 6 modules waiting for their Drupal 7 upgrade) ...

For all these reasons, I will continue to go for Drupal 7 for quite a while (probably at least the rest of 2015). Because of its maturity, and because I would not want to depend on a Drupal 8 release of either core, or any of the contributed modules that I "must have".

There is also that great presentation from David Reid about all those amazing contributed modules in Drupal 7, which are like backports of new Drupal 8 features.