Run dpkg-reconfigure locales. You will be presented with a screen where you can choose which locales to compile. I mainly use US English but I use Swedish too, so I only choose the three (in my case) en_US variants - iso8859-1, -15 and -UTF, and the same encodings but for the Swedish locale (sv_SE).
I suggest you do the same. If you will only use English on the system you can safely choose only the three en_US locale encodings.
ro_RO, which seems to be the culprit behind your error, is Romanian and if you don't usually use your system in Romanian there is no reason for you to enable it in your locales config.