@montezh2001 -
You can definitely go ahead and open a ticket but let me elaborate more on how this works.
It expects the ISO currency code as the parameter (e.g., ‘USD,’ ‘EUR’), but there is no documentation around it.

I tried on Postgres (strings, doubles, etc., for both <amount> and <currency> )
but got the cast-type errors:
PGS-220000 PostgreSQL error: ERROR: function formatcurrency(integer, integer) does not exist LINE 1: ...iId,dPrice,sCurrency) SELECT W0.iId, W0.dPrice, FormatCurr... ^ HINT: No function matches the given name and argument types. You might need to add explicit type casts.
The currency is the one listed in srcSchema currency.xml (in the type enumeration)
You might try on Postgres or oracle, but you will mostly get an error.
funcList.xml only shows a definition for SQL Server.
In fact, because of the current limitations, we should have the "FormatCurrency" function removed/deprecated in the future.