Some benchmark surveys ask for spending data in U.S. dollars and leave it to the participants to convert their non-dollar spending however they choose to do so. Other surveys, including GC Metrics, accepts data in whatever currency the participant uses and then has to decide on a conversion rate.
What I have done is taken the approximate average of the currency against the U.S. dollar for the calendar year involved. By approximate I mean that I eyeball the exchange rate for the year and pick a figure that seems as representative as possible. There are undoubtedly more precise ways to convert currencies, but they would be much more computationally intensive and harder to explain to those who receive the report.
Take the GC Metrics 2013 survey; https://novisurvey.net/n/GCMetrics2013.aspx. The no-cost survey asks for 2012 number of lawyers, paralegals, and other staff; inside and external legal spend; and revenue. You will receive the Winter Release with more than 1,100 law departments.