With this Python code you can convert between currencies based off on EUR.
Get your own api from https://exchangeratesapi.io.
You can use the free version to get 1000 monthly requests.
I suppose free plan only supports EUR based conversions.
I believe you can increase the number of currencies you want to convert from EUR, you basically need to add your currency in the api after the "symbols=" section with comma seperated. (I only tried to add JPY which worked for me.)
If that works you can also make the user add currencies from input. However that would be harder to control since the user can always enter false arguments resulting in the error of code.