D8 How to create custom configurational form translatable?
You need a schema yaml file in the module folder /config/schema to make configuration forms translatable: 


  What are schema files used for?
  
    The primary use case schema files were introduced for is
  multilingual support. We need to have a tool to identify all
  translatable strings in your shipped configuration so when you ship
  with your own settings as well as default Views, additional user
  roles, menu items, etc. we can offer those up for translation as part
  of your module/theme release on http://localize.drupal.org. The
  nesting levels and types would be enough for this use case.
    We also use schemas to provide actual translation forms for
  configuration based on your data. This use case is where types gain
  more importance and labels become crucial. The core Configuration
  translation module uses schemas to generate translation forms and
  save translations. The two most important built-in translatable types
  are 'label' for one-line text input and 'text' for multiline text
  input.


Source: https://www.drupal.org/node/1905070