Confirmed.
The English localisation files in the Poland DLC are OK, correctly giving:
POL_CAS_equipment_1:0 "PZL.23 Karaś"
POL_CAS_equipment_1_short:0 "Karaś"
This is an issue only on that interface bottom/middle - it's OK top right of the air wing details, and in research. It's a different font on that part of the UI to everywhere else. Other Polish planes using accented characters probably have the same issue.
Can't tell if the same issue might occur on the ship details UI, the examples in-game on 1.1.1936 don't have accented characters.