Depending on the document i.e. Thai government documents in English, or Thai, or Certified Translation etc. it can also be "legalized" at Thai foreign affairs office at MBK, 5th floor, Section A. 200 Baht 3 day service, 400 baht same day express service. You can make an appointment in advance on their website, can google it and use the appointment QR on their page. I don't have their website with me at this time. They are open at 10:00 AM-04:00 PM.