Hi Marzanna, cool you are beginning to write and apply self-forgiveness. The language depends on you, the point is essentially writing in the language where you can ensure you can explain/describe the points effectively for yourself, so it's not necessary to 'write in english' if you don't want to and are comfortable doing so in your native language. However, if you'd like to cross reference your writings, then yes English is the suggestion so that you can share here at the forum.
This question, If I recall, was answered to you or someone else in a DIP Hangout, so yes it all depends on you as what matters in self-forgiveness is your ability to see yourself/describe yourself as your mind and understand the point of self responsibility laid out as self-forgiveness, as well as the corrective statements and commitments to apply for actual self-change.
That's my suggestion, and enjoy supporting yourself.