Best beloved bookstore customers,
Please do not ask me how to say thank you in "my language." Not even if you are a dapper elderly man who I assume is only trying to be charming in a rather misguided way. I think that you will be disappointed to discover that thank you in my language is mostly similar to thank you in yours, with maybe a few small variations along the lines of thanks, or even thanks so very much.
This would be because I speak English, like you, and nothing else, except for a small grab bag of forgotten French.
So, merci bien! And, if it will make you happy, I suppose I could throw in an arigato or two.