When something comes out of left field, it is comes from an unexpected place or direction, or is surprising or unexpected, and something that is left field is uncommon, unpopular or strange [source].
This phrase comes from baseball and refers to the left side of a baseball field, although why something coming from this part of the field is unexpected or surprising is uncertain, according to Know Your Phrase.
Another way to say that something is unexpected is that it came out of the blue, which apparently comes from the phrase a bolt out of / from the blue. This refers to the unlikelyhood of lightening coming out of a clear blue sky (another version of the idiom).
A bolt out of the blue was first used in writing in The Standard in 1863, and out of the blue first appeared in The Spectator in 1879 [source].
In French an equivalent of like a bolt out of the blue is comme un cheveu sur la soupe (like a hair on the soup) [source], although I don’t know why. A hair in your soup is more likely than lightening from a clear blue sky, I would think.
Are there interesting equivalents of these phrases in other languages?