This one is one of the bigger surprises in drupal 8. Form building is straight forward, basic OOP bases on symfony, but then you want your forms to be translatable, you are in for a ride. First you will need to have schema yaml files in install and schema directory. After that you will also need to add my_module.config_translation.yml in route of your folder with some repetetive info and then in the end to have a local tab appear on your form, another yaml file my_module.links.task.yml as you need to have at least one local task in your path for translation tab to appear.
To have more info and how to do it step by step, look here
or some more basic info about forms here