Hi, The problem was due to some faulty KDE Plasma settings.
While probing around, I noticed that even a simple "LANG=C glabels-3" generated error messages about missing libraries. In addition, it turned out that glabels runs fine under a new (= freshly created) user account. Practical solution: quit KDE, rm ./config/plasma* and log in again. You have to re-configure a few KDE settings, but at least the locales are working now. Cause: unknown, most probably some weird KDE setting that was carried over throughout the years and messed up with the recent migration to Plasma5. Solved, case closed. Thank you. And ... stay safe.
