Thank you shg,
finally got it. I had issue with the translation. It is related to east asian language support and I obviously never installed it. The shortcut is ctrl+shift+J (quite undocumented). Made a few tests and seems not to work perfectly on multy row paragraphs in italian: it still stretches spaces instead adding additional spacing (in other words spaces do not have discrete width). This way characters are not on same "column" and document has no clean look. I will try a bit more may be there is something i'm still missing.
Thanks again