On Windows, Kleopatra ignores the language settings made in Kleopatra itself and instead seems to use the language configured for the system.

The screenshot shows the filters with English language configured in Kleopatra and Windows system language German.

Another screenshot with KF6 version and German in Kleo with EN Windows system language:

This is after restarting Kleopatra, of course.
For all filters defined in the libkleopatrarc the system language setting is used, for the others the language used is the one configured in Kleo.
Probably a kconfig issue.

Related ticket: {T3733.} That one is for GpgOL/EX