german translator here:
balooctl has a Messages.sh and extracts 52 messages into the catalog balooctl5.pot
a lot of messages in statuscommand.cpp, main.cpp, monitorcommand.cpp configcommand.cpp 98 - 52 = 48 are not extracted due to missing i18n() calls
we translate description of cammandline options in all application, therefore we should translate all missing messages here as well

I see. Thank you for the info.
The only thing translated in this function is the filename. I am new to this, but I think that is the only item that does not need translation.
It should be exactly the other way round, right?
Regarding my patch:
To ease translation, should I break up the strings a little bit?
Like

out << i18n("Basic indexing") << " " <<i18n("done");

No. Please have a look at https://api.kde.org/frameworks/ki18n/html/prg_guide.html

The only thing translated in this function is the filename. I am new to this, but I think that is the only item that does not need translation.
It should be exactly the other way round, right?

"File: %1"will be translated e.g. in german to "Datei: %1" and at runtime %1 is replaced by the string value of filePath

To ease translation, should I break up the strings a little bit?

Please no, word puzzles are evil for translators

Just installed baloo5-lang. Now I see the difference

If we want output of balooctl status <path> to be parseable, then IMO it shouldn't be translated.
However the translations in the status command without arguments are instead desirable IMO.

<filename> ... overkill?