Normal C strings don't support unicode, so doing QString("\u2060") will give a compiler warning saying "C4566: character represented by universal-character-name '\u2060' cannot be represented in the current code page (1252)", and will replace the \u2060 characters with question marks.

Instead, it works to do:

QString("Défaut suspension: Limitez votre vitesse à 90km") + QChar(0x2060) + "/" + QChar(0x2060) + "h"

Or, if that's too difficult to read, this also works:

QString("Défaut suspension: Limitez votre vitesse à 90km\1/\1h").replace('\1', QChar(0x2060))

[SOLVED]

Eventually, the solution is to add escape character '' before any special character which has a unicode between 161 and 255 (including).

E.g. "abékl" -> "ab\ékl"

When looking into Qt source code and specifically to int QCssScanner_Generated::lex() (qcssscanner.cpp) one can find a huge state machine parser (css based) which expects the escape character before unicode characters 161 to 255.
(the parser get called by QWidget::setStyleSheet eventually)

The reason Japanese, Chinese and more non-Latin languages worked is because the state machine handles unicode characters from 256 above as is (does not expect the escape).

Hope this will help others.

p.s.
Using QString::fromLatin1() didn't help.