Steps to reproduce:
1. Create a new node in English
2. Create a translation in French and add FR or something else obvious to the title
3. Edit the node in English
- note that the title is now the one entered for french.
- Look in {field_revision_title_field} and note that the title is the new French one for both the en and fr values.
This is because when new translations are saved from the entity translation UI, $node->title gets set to whatever the translated version is going to be. title_active_language() return 'en' during title_entity_presave(), so the English field value gets overwritten with the translated title.
I've got a patch which fixes the issue for me, this appears to work fine with the UI but there's one edge case where it'll break.
If you load a node, change node->title, then also add a new translation, then it woudn't update the title - but that's only possible via the API rather and seems unlikely.
Uploading patch once I've got an issue nid.