When doing business in Thailand, presentations can be fully prepared in English to deliver to Thai businesses, especially in Bangkok. However, emphasis should be placed on clearly enunciated speech and presentations should avoid complex language as it will embarrass the people you are presenting to if you lose them with overly colloquial terminology. As such, Thai people are generally comfortable with British and American speakers while other nationalities and accents can sometimes cause some trouble. But as stated above, always keep in mind the need to speak slowly and clearly and things should be fine. On another note, don't be afraid to take an interpreter if language poses to be a problem.
In terms of the actual deliverables, make your materials colorful and full of photographs and other items that communicate the product or service being presented. Try to avoid large black and white tracts of text, especially if it is in English. Be careful also of losing your message through lack of content. Make sure to have lots of copies of your proposal—ideally one for each person attending the presentation and a few spare for staff unable to attend, but who may be interested.
For legal documents, you can also be confident, especially in larger companies and definitely in Bangkok, to use English as the language for all of your proposals. Contracts in English are almost always acceptable.”