You can then calculate what the price is in your currency. If you can speak some Thai you will receive in most cases a prompt response in Thai or English. Thais respond to foreigners more so when they try to speak the Thai language. In many cases they will help you with the proper pronunciation. If they respond in Thai and you don't understand, hand them the calculator and they will show you the price.
Many vendors have their own calculators.
Vendors remember customers who visit them frequently. Some are very friendly and want to know about your home country. Some want to practice their English by talking to you. Over the years we have become friends with many of the vendors that we've dealt with. You can bargain the price.