the following issue may be related, but is in fact different to #1545896: Add Entity Translation integration
Before explaining the issue, here's my setup:
My node type is Entity Translation enabled and has a Entity Reference field with Inline Entity Form as widget, allowing an unlimited number of values. Due to the lacking ET support, I've set the field itself to be translatable, but not the entities. So that every node translation has its own set of inline entities.
Adding, editing and removing entities from existing nodes works perfectly, thus changing the node language afterwards is not recommendable, but that's no problem for me.
But when I created inline entities at the same time as the parent node, I've experienced the problem, that those entities get the wrong language code set (neutral). E.g. you create a new node in German, and while not having it saved to the database, you add one or more entities to the node. As the node doesn't exist at the time of saving the entities, they and especially the entity reference field values all get the 'und' language code. So, after creating the node, the node is having a different different language (german) than the entity reference field values (neutral), which is leading to the problematic situation, that you still can view the fields, but no longer edit them on the node edit form because they are no longer appearing there.