Recently I published an android app for conversion calculation of Croatian Kuna (HRK) to Euro (EUR) and vice versa, using the fixed exchange rate of 1 € = 7.53450 kn.
The app is free and currently contains no ads. It can be downloaded from the Google Play Store at the following URL:
https://play.google.com/store/apps/details?id=com.eurokonverter
It has a dedicated web page here:
https://eurokonverter.com
Oh, and the app is available only in Croatia. ;-)