Suitable visa for me

Hi. 

I'm 26 year old, I have Indonesian passport and currently living in Melbourne, Australia as Working Holiday Visa holder. My situation right now requires me to go to Germany for an adoption procedures, I will be adopted by a German parent. 

I've never been to Schengen area before, i need someone to advise me the suitable visa that i need, visa requirements, the duration of time that allows me to stay, and other things that i need to know.

I will need to sign the adoption paper by myself and the german adoptive parent in front of the notary, then in an undefined time frame will need to attend the court for the agreement of the adoption by the judge. Therefore i need a longer time period to stay within Schengen area.

My plans along with the adoption procedures are enrolling to German language course for 6-12 months before continuing my study in university, then work and stay in Germany. 

I've searched for an Au-Pair program. It can be an alternative visa, which it allows me to stay up-to 12 months in Germany. But the thing is, I don't know whether this visa allows me to do some professional law related procedures in Germany. I need help and advise on this and on some more visa alternatives that suitable for me. 

Thank you for the help