How can I soundproof a grille in a wall?
Soundproofing and ventilation are not compatible.  The louvers could be blocked/unblocked at will with a rectangular piece foam rubber that is several inches (10cm or so), for example from an old sofa cushion.

Insert it when you want privacy is desired. Take it down, or pull it back otherwise.  You may find that you don't notice any difference in air quality inside the room, and end up leaving it in place all the time.  Or you may find that the louvers allow heat into the room in the winter.

Cut it to size + a couple centimeters.  If cut too big, it may bow away from the louvers.  If too small, it may not have enough friction against the walls to keep it in place.



Larger cities have stores dedicated to selling foam rubber or different dimensions and densities.

Thick foam rubber is also available from repair shops that re-upholster furniture.

Looks like people are selling foam rubber on ebay:

http://www.ebay.com/sch/i.html?_nkw=foam+rubber

Per @longneck's comment, foam rubber can be purchased at craft and sewing stores.

A sufficiently thick pick can be built by laminating together thinner pieces, say 3 or 4 layers of 1", using double stick tape for example.