Block configuration form values are not translated
You have to provide a schema in /config/schema/mymodule.schema.yml of the type block_settings.

Example:

block.settings.example_configurable_text:
  type: block_settings
  label: 'Example configurable text block configuration'
  mapping:
    block_example_string:
      type: text
      label: 'Block contents'


From the example module http://cgit.drupalcode.org/examples/tree/block_example/config/schema/block_example.schema.yml

Configuration values you map as type: label (single line of text) or type: text (multiple lines of text) are translatable.