In Qt you can use %L1 to get the localized representation “123,456” of the number, as %1 gets the normal “123456” version of it. Check: http://doc.qt.io/qt-5/qstring.html#arg-2 This is a big addition that non-latin languages can benefit from. Not sure how that can be coded though...
According to https://api.kde.org/frameworks/ki18n/html/prg_guide.html ‘Integer arguments will be by default formatted as if they denote an amount, according to locale rules (thousands separation, etc.)’. So it shouldn’t be *necessary* to write %L1. But using just %1 currently doesn’t work, cf. the discussion at https://phabricator.kde.org/D13219. I think this is just a bug.