The Queen visited Google's London headquarters today and watched a YouTube video of a baby laughing, which has been viewed millions of times. An unseen adult says 'boo', which sets the baby off into fits of giggles. The video is Swedish, so presumably the Swedes say 'boo' to frighten someone, just as we do in English.
I'm constantly being amazed by the similarities there are between languages, so I thought I would see if I could find out if other languages used 'boo'. I speak French, German and Russian, but I don't think I've ever been in a position where I've needed or wanted to say 'boo' to a native speaker. When I have played with French, German or Russian babies, I have made English nonsense sounds to them, hoping and assuming that they wouldn't mind too much.
As far as I can tell from looking in my dictionaries and on the internet, several European languages (I don't know about others) have a word that sounds like our 'boo' to use in situations where one person creeps up to another and frightens them by making a sudden sound. French has bou, German buh and Spanish, Italian and Portuguese bu. My Oxford English-Russian Dictionary says 'boo' is y-y (pronounced 'oo-oo'), but I'm not sure about that as it's got two syllables, which is probably too long for a word designed to give a sudden fright.
In English the onomatopoeic word to represent the sound of crying is 'boohoo'. French and Spanish prefer snif snif and German has schluchz schluchz. In Russian, according to my dictionary, it's y-y-y (pronounced oo-oo-oo). Those words may not be identical to the English version, but they are very similar in the sense that they are reduplicated words (a linguistics term meaning the second syllable is a repeat of the first, or rhymes with it).