If spending enough time in a particular country, take the time to at least learn your numbers in the local language. Negotiating in the local language will always land you a better price and can be a lot of fun! Even if you mispronounce words, the effort at least shows that you are interested in the local culture rather than just buying cheap goods to take home.
An excellent way to practice your numbers is when ordering food; have the server repeat the menu item number for you when ordering. When paying, say the price in the local language to the cashier. Practicing this several times a day will have you counting in less than a week.

