just go to your local Amphur 'district' office and ask what documents you need for them to marry you, my wife and I just married in the presence of two witnesses
You will need the CNI and it will need to be translated to Thai. The translation needs to be done by anybody who is recognised by the ministry of foreign affairs as certified translator
No a district office or Amphur will only issue documents in Thai, to get it translated contact the ministry of foreign affairs in your province. Make sure who ever translates it is recognised by the ministry and has a stamp of certification