Translate tab missing on custom config form
Issue:
You have created a custom Drupal configuration form but no Translate tab appears when you access it.
The config_translation module automatically derives the Translate tab from the configuration mapper. However, Drupal only renders the local task (tab) navigation when there are at least two tabs on the same level. Since the Translate tab is the only local task available, Drupal does not display the tab navigation, causing the Translate tab to remain hidden.
Prerequisites:
You have the config_translations module enabled.
Solution:
Drupal Core solves the issue for settings forms by providing a default tab. We apply the same approach in our module by adding a default local task, which allows Drupal to render the tab navigation and display the Translate tab.