There is a large media scandal in Romania about a car that had a preferential number from another country. According to police, the number was illegal because was having letters only and no digits so basically, this number was violating Vienna Convention on Road Traffic (1968) (wikipedia link here) that was ratified by Romania in 1980.
Now according to a UN official that convention was modified in 2016 and because Romania did not oppose it is should be automatically applied. The amendment allows letters only plate. Is there any UN law/directive that says that a country that doesn't oppose an amendment is accepting it by default?