I noticed another problem today, but I can not quote the error message because I returned the site to a previous backup.
I had a default post content type with the setting "Multilingual support" "Enabled" but not "Enabled, with translation".
I created Post-type content in Bulgarian language. Then I changed "Multilingual support" from "Enabled" to "Enabled, with translation".
And when I tried to translate this post from Bulgarian to English it was impossible, it appeared error message that there was no some table connected to module i18 - probably something from the old Drupal code. Moreover, even editing of the Bulgarian post became impossible and the same error was displayed.
I returned the site to a previous state from an backup archive where there was not a single post of the post content type. I made a settings of the default post content type even before I publish anything of this type to be "Enabled, with translation".
Then I created a post in Bulgarian, translated this post into English, and everything worked as expected and error-free.
The problem I see is that you can't change "Multilingual support" from "Enabled" to "Enabled, with translation" if some content of that type has already been created.
This can be very unpleasant if a significant amount of posts of a certain content type have already been created and the customer later decides that they want multilingual support and translation of existing content.