When I applied for my wife's settlement visa we submitted some of the documents in Thai and never had a problem is that still the case that you can submit some documents in Thai for example house books.

Fourth attempt to post this; hope it's the last!

Currently the Guide to supporting documents says that documents not in English or Welsh must be accompanied by a translation certified as per the guide.

However, posts on this and other forums suggest that it is not always necessary for official documents.

Up to you if you want to risk it.