Checking language selection for programs started by “Dolphin”

English Applications
1

I can open a text file from a directory display of the application “Thunar 1.6.12-122.2” with the application “KWrite 17.04.3-1.1” as expected. The text editor displays its user interface with the primary language.

I can try the same action from a directory display of the application “Dolphin 17.04.2-2.1” (by a double-click within a XFCE session). But I wonder that a secondary language is displayed then by this editor. The secondary language is also activated somehow if I select the entry “Open with” from the context menu.
How can the primary language be displayed there by default?

2

This is probably caused by an incomplete set of language files for KDE apps. Check whether bundle-lang-kde-$YOUR_LANGUAGE is installed.

3

This is the case for the languages I am interested in for the affected graphical user interfaces.
Any further ideas for checking corresponding settings?