I noticed that some users of Interest Calculator had problem in changing Settings (such as Language, Currency and days of the year).
So, I submit some screenshots, directly from my iPhone (sorry for quality, and for Italian), to give you all the information about changing iPhone Settings ;)
Here is the method:
1) go your iPhone main menu:
2) scroll applications into iPhone Settings, until you find "Interest Calculator"
3) change your "InterestCalc" settings
I hope this small howto can be helpful for you. However, If you have problems, please contact me, or leave a comment ;)