In Europe they are using the metric system and for example bra band sizes use 5 centimeter increments, instead of 2 inches. That being said they are very close. All products are marked with US size equivalents and in 99% of cases customers buy the same size they are accustomed. What really matters is that different styles fit differently, and brands are always a little different from one another, just like with US brands.
In France items that are made in Small/Medium/Large etc. sizes shift down one size when adjusted to USA sizing standards. In other words what is a Medium in France would be considered a Small in the USA. Therefore, certain items from French companies that are made only in Small, Medium and Large will not have a USA Large option available.
There are several different methods you can go by. If you have equal proportions top and bottom you can follow this guideline: 30 inch bra band size/XS panties, 32/S, 34/M, 36/L, 38/XL and so on. If you are a little bigger or smaller adjust accordingly. OR another option is to go by dress size" 2 - 4/S, 6 - 8/M, 10 - 12/L, 14 - 16/XL, etc.
Feel free to contact me if you have any sizing questions.