I use blocks in a panel with Drupal 8. With view generated blocks there are no problems: The block titles are translated accordingly to the view settings. With custom blocks, it appears to be problematic to get the block title translated. I hoped that these titles would be translatable via the user interface translation but they are not; plus, in the panel settings, I can set the block title, but I cannot translate it.
How can I translate block titles (and maybe block content) of custom blocks in a panel, with Drupal 8?