A new 'Restriction by language' availability condition additional plugin for Moodle 2.7 (available from https://moodle.org/plugins/view.php?plugin=availability_language ) looks very promising for all bi-lingual and multi-lingual sites.
It might be a good idea to try it in as many sites as possible, and if it is as awesome and well implemented as David Mudrák wrote, I'd suggest that we add an issue in the tracker and vote for this plugin to be included in future Moodle core branches.
I will be looking forward to the opinions of other Moodle translators.