Multi sites: should I use Craft 2 locales, or go with Craft 3 alpha?
Brandon has stated (on the CraftCMS Slack channel, so, sadly, not easily linkable here) that custom locales will automatically translate to sites when upgrading from Craft 2 to Craft 3.

At this point, Craft 3 is still in developer preview and not recommended for production sites, so I would recommend developing in Craft 2 and updating when a stable, final version of Craft 3 is released, along with any compatible versions of plugins that you need for the site(s).

Due to the changes in Craft 3 (many of which are still in flux) plugins will need to be re-written to be compatible with it. Depending on the developer and the complexity of the plugin, this may take some time.