Use System locale for Defaults

[flywire]

Should use system locale for all monetary defaults rather than just 3-letter International currency symbol (#174) with rest from https://github.com/sdementen/piecash/blob/master/piecash/core/currency_ISO.py

[sdementen]

Sometimes you may need to create a currency that is not in your locale for foreign transactions. Would you advise here to play with locale to get these currencies too? Isn't it more complex than just giving the iso of the currency? Via the locale, can you recover the information that is in the currency_iso.py file?

[flywire]

I tested locale in on online python box and it generated exceptions because locale wasn't available. I'd expect setting locale as default would use all system settings. If an actual currency is specified or recovery after an exception then use currency_iso.py file.

Yes it is more complex. First priority would be just roll out 3-letter International currency symbol.