I think I have found the related source file and can add it to the search path for generating the translation files. Should this go into the stable or dev branch?

I have already searched (but not very deeply yet) in the Qt sources for the occurrence of the "Search" string in order to include this file into the scan while updating language files. This is already done for some Qt headers for languages that do not have a Qt translation.

The text for About Octave is not static at compile time so that is why it is not being translated.  See libgui/src/main-window.cc

  void main_window::show_about_octave (void)
  {
    std::string message
      = octave_name_version_copyright_copying_warranty_and_bugs (true);

    QMessageBox::about (this, tr ("About Octave"),
                        QString::fromStdString (message));
  }

The call octave_name_version_copyright_copying_warranty_and_bugs() is to a function located in liboctave/version.cc which does some more string manipulation.

std::string
octave_name_version_copyright_copying_warranty_and_bugs
  (bool html, const std::string& extra_info)
{
  std::string sep = (html ? "\n</p>\n<p>\n" : "\n\n");

  std::string msg;

  if (html)
    msg = "<p>\n";

  msg += octave_name_version_copyright_copying_and_warranty (html, extra_info)
         + sep
         + octave_www_statement (html)
         + sep
         + octave_contrib_statement (html)
         + sep
         + octave_bugs_statement (html)
         + (html ? "\n</p>" : "");

  return msg;
}

Potentially, there could be two strings which are static at compile time, one in plain text format and one in HTML format and then the code would have to be changed to call the right one.  But, part of this is hooked in with the GNU build system which I believe auto-generates version.cc.

My thought is that getting this translated would involve a lot of pain, for very little gain.  Thus, I'm inclined not to fix this instance.

The language were updated with cset http://hg.savannah.gnu.org/hgweb/octave/rev/0bec36b8a619 on stable, at least with respect to the context menu entries in the variable editor. An update with the strings of the search pane in the doc browser is still missing.

Reopening the report

Well, this bug is closed but the language files themselves still need updating.

I didn't note yet that the .tts files have been updated by Torsten, I'll make the Dutch translation soon (the updates were made on default (AFAICS)

The first case in the variable editor is probably because the tr() macro which marks strings for translation requires static strings rather than a variable piece of code.  I made this change.

diff -r bb3a441addb0 -r 1ff470c9e9c2 libgui/src/variable-editor.cc
--- a/libgui/src/variable-editor.cc     Sun Feb 24 21:22:49 2019 +0100
+++ b/libgui/src/variable-editor.cc     Sun Feb 24 22:02:04 2019 -0800
@@ -668,7 +668,7 @@ namespace octave
       }

     QString column_string
-      = tr (column_selection_count > 1 ? " columns" : " column");
+      = column_selection_count > 1 ? tr (" columns") : tr (" column");

     QMenu *menu = new QMenu (this);

@@ -715,7 +715,7 @@ namespace octave
         rowselection_count = 1;
       }

-    QString row_string = tr (rowselection_count > 1 ? " rows" : " row");
+    QString row_string = rowselection_count > 1 ? tr (" rows") : tr (" row");

     QMenu *menu = new QMenu (this);

The partly translated strings in VE are caused by a string that is composed by "cut" and a variable part "row", "rows", "column" and "columns". The translation of this variable part obviously does not work. This is already fixed, but I am still trying to update the language files.

The dependence of the file browser translation on the settings file is really weird. Does this happen on Linux (KDE?) or Windows?

Torsten,
After renaming .config/octave/qt-settings indeed the file browser headers became translated.

On the one hand that is good, OTOH it's fishy as AFAIU the qt-settings file should be upwards-compatible. Or am I wrong here?

Anyway what struck me (but I didn't mention) is that the V.E. context menu texts were partly translated: e.g., "Ausschneiden column".

Just a few cases I hit last week:

Variable editor
When right-clicking on a column or row header, I see in e.g., Spanish translation terms like "Cortar column" and "Copiar row", resp. Similar holds for Dutch and German translations.
Text in these VE context menus e.g., "Cut column" and "Copy row" etc. don't show show up in any .tts file.

Documentation pane
"(e.g., German, French) "Suchen" or "Recherche" tab: the text to the right of the search text box mentions "Search"

File Browser
The column headers mention "Name", "Size", "Date Modified" etc. while in the context menu one gets after right-clicking the column headers, the translations are shown.
In the Workspace pane the column headers are translated so I suppose it should be possible to get it right.

No big deal I guess but is might appear a bit sloppy to unwary users.
I'm prepared to update translations quickly but maybe wait for the 5.<whatever> bug fix release?