Next try:
https://commits.kde.org/kate/1e54bfca9028ad15eb83e125a242ac35334e1904

Where can I find documentation about that "context" ?
In the docs for i18n(), i18np(), i18nc() etc. it is just mentioned that there
is a context parameter but not what that is or how it should be used.

First try to fix it:
https://commits.kde.org/kate/bd321190aedfcd4a3a19f215abf13cd3e135138f

This is the call:
i18np("<b><i>One match found in folder %2</i></b>",

"<b><i>%1 matches  (%3 checked) found in folder %2</i></b>",
  m_curResults->matches,
  m_resultBaseDir,
  checkedItemCount));

Done :-)
https://commits.kde.org/kate/749fa3b25f0b26c61e0c15004d490d77ce21dea0

Kare, what do you suggest ?
How about committing that now (and make the exclude-filter work), and after that do the refactoring ?
I can give it a try.

In D9901#191667, @sars wrote:

According to my tests build* does work as expected, but build/* does not work. I'm actually a bit surprised that the latter works in "Project" mode.

Basically this change would make it possible to exclude only directories with a "xxxx/*" pattern, at a the cost of a bit slower generation of the search-files-list as it would always iterates over the whole path+filename.

Definitely an improvement :-)