I'd create a key to link the tables and create a new Month field in the rates table that includes the year too. That new field RateMonth is where you make your selection in for the rate to be used for the rate conversions.
Thank you very much for your answer but i'm not sure it works.
UK_January will be always converted using the rate of January for UK even if you launch a situation at March, or July or August.
What I would like is UK_January converted with the rate of GBP for March, July or August when I launch a situation at March, July or August, Sweden_January converted with the rate of SEK for March, July or August when I launch a situation at March, July or August...