@Arun Kumar, thank you, it works.
Didn't realize the locale name is wrong. However, I wondering, why I cannot easily use the localization string {$ membership.passwreqsubject $} which should do the same thing, as per documentation?
Why the default locale used instead of website default locale?
Also, the second issue - I still getting the email with wrong subject, "??dost o zm?nu hesla" instead of "Žádost o změnu hesla". Perhaps, you can give me a suggestion, how to fix it?