How to displaying translate tab in the node
This tab does show when:


You have more than one language installed. Locale module needs to be enabled.
You have Content Translation or Entity Translation modules enabled (not i18n)
You have set up the content to be translatable, in the the content type settings page (if it's a node). When using Entity Translation, prior to that, you should define the entity type (e.g. node) to be translatable.
Your user has permissions to translate content.