Below you can see Krusader in current Tumbleweed Plasma 6 and from remote Leap 15.4 Plasma 5. You can notice that half of the strings show up in English despite all settings set to Czech. The remote Leap 15.4 has all in Czech.
Please note that due to other issues, that happened already before the upgrade in Leap 15.4 and which prompted me to try the upgrade, I had to delete some of my plasma settings and let them be created again in the Tumbleweed.
I have also tried a completely new user. This is how he sees the main menu:
And this is how the new user sees the aforementioned Krusader menu:
OK, so a lot has improved by installation of various -lang packages that for some reason were not included in the upgrade (e.g., krusader-lang]. Someone on a different forum mentioned that a fresh install caused similar issues for them.
Not everything has been fixed, though. Kate and Kwrite still have these problems and I cannot find their -lang packages in the repositories. The questionmarks in Yast are also still present.
Do you have the locale properly set up? During the update from KDE Plasma 5 to Plasma 6, zypper decided I don’t need any locales anymore so the entire desktop was a mess of english and czech.