PHP Notice is given due to deprecation of wp_get_sites() in Wordpress 4.6. Should now useget_sites(). Please see ticket here, https://core.trac.wordpress.org/ticket/36994
Is implemented in the source - 1856b7e
As @bueltge already pointed out, this is already included in the master, but not shipped in the latest patch update (i.e., MultilingualPress 2.4.7). wp_get_sites() does still work, and due to our policy of staying backcompat with two major versions back, the change is (in my opinion) too much for a patch release. The function didn't just get renamed, it also returns completely different things now.
Use `get_sites()`, where available. #229 #236