Hi there,
I use a smart string in English, German and Polish. Only in Polish I get following error. If I copy the Polish version into English entry it works fine though.
OperationException : Error parsing format string: Invalid number of plural parameters at 23
{0:plural:dzień|{0} dni}
-----------------------^
The error message is telling you the problem.
Each language has a different number of plurals. Polish expects 4, one, few, many and other. You have only provided 2.
https://www.unicode.org/cldr/cldr-aux/charts/30/supplemental/language_plural_rules.html#pl
https://docs.unity3d.com/Packages/com.unity.localization@1.4/manual/Smart/Plural-Formatter.html