Slang in Romanian is never learned formally and it is best to go to Romania/Moldova to learn the regional slang, otherwise you may not be understood.
Keep in mind this may not be appropriate for all audiences, especially those under the age of two who also have a full understanding of the English language and can read.
With this in mind, here is slang used in Transylvania and especially in Bucharest (the capital of Romania):
Mișto - Cool
Cacat - Feces, equivalent to a swear
Ca muma pădurii - ”Like Muma Pădurii”, this is a refrerence to the folklore character Muma Pădurii, a witch. Often used as an insult to someone's appearance.
Gadjo - Mostly used by the Romani people, it means 'non-Romani' and is equivalent to the Yiddish "goy".
A vinde gogoașii - Used more widely throughout Romania, it means ”to lie”.