Thanks for the clarification. I always wonder about capitalization myself and finally decide on whether I think it's a complete phrase -- "Thank you." "You're welcome" -- or part of a sentence -- "I want to thank you for the gift." Here, it sounds like a simple "Thank you," so I would probably use it in that form.
I wouldn't capitalize it. If "thank you" represented a sentence that you were quoting, maybe. (For example: In response to my help, she said, "Thank you.") But all you're doing here is transcribing a phrase, so I'd write it lowercase.
Lower case 'thank you', no internal punctuation, as it's just being treated as an expression. Some expressions, such as 'hello', 'yes', and 'thank you', are commonly used as complete utterances, but you're not treating it that way: there's a sign for 'rabbit' and a sign for 'thank you' and a sign for 'Japanese', and so on.