Bug 459438

SUMMARY
Okular doesn't use system/printer settings; we use British & American English and set some (locale, programs) locale to British English, but when I try to print from Okular, despite my PC (British English locale) and network printer settings' both are set/overrided to USA letter print-format, Okular preselects an International Standards Organization (ISO) letter print-format: A4 (British Isles & ISO/mostly-worldwide), and other family PCs (past British, now USA English locale with  only date/clock format override) have/had mixed language settings same behaviour (we occasionally use some French, German, Spanish, Russian, Greek locale/software keyboards).

STEPS TO REPRODUCE
1. Set mixed locale system/programs/printer: PC whatever but override printer to different dialect locale letter print-format (different shape/size).
2. Set Okular to system dialect (didn't automatically use mine... minor error many/most/all KDE programs have; I'll maybe report later).
3. Open Okular print menu for any text/ebook.

OBSERVED RESULT
Uses neither default PC letter setting nor (network) printer's default.

EXPECTED RESULT
Preselect use PC/printer letter setting: at least your PC's, if not (network) printer's (if different, I don't know).

SOFTWARE/OS VERSIONS
UNIX/GNU/Linux/KDE Plasma: Slackware 15 (stable, current) & KDE Neon 5.25 GNU/Linuxes (presumably all OS with Okular 20+... can check most/all main *BSD on request)
KDE Plasma Version:  5.25.5
KDE Frameworks Version: 5.98.0
Qt Version: 5.15.9

ADDITIONAL INFORMATION
I've never printed A4 with current printer nor this KDE/Okular configuration (erased/restarted .kde*, .config/(k*|okular) Spring 2022).