As I am running some internal spreadsheet to track our cash flow, I had to find out a solid foreign exchange rates and currency conversion API. I have found that Fixer.io provides access without any cost for up to 1000 APIs calls per month which was enough for my needs.

Fetching foreign currency rate for pair EUR/PLN using this API looks like this

http://data.fixer.io/api/latest?access_key=XXX&base=&symbols=PLN

and we will get an output as

{
	"success": true,
	"timestamp": 1595759045,
	"base": "EUR",
	"date": "2020-07-26",
	"rates": {
		"PLN": 4.405059
	}
}

Injecting this currency rate directly into Libreoffice Calc spreadsheet will look like as below as we will use WEBSERVICE

=WEBSERVICE("http://data.fixer.io/api/latest?access_key=XXX&base=&symbols=PLN")

but then if you would like to extract for your computational need just the rate you might need to use regex

=REGEX(WEBSERVICE("http://data.fixer.io/api/latest?access_key=XXX&base=&symbols=PLN"),"\d{1,2}[\,\.]{1}\d{1,5}")

where using \d{1,5} you specify amount of decimal places you are interested in of number 4.405059