This one should translate very well into Russian -- only I'm not sure about one particular word:

Two five-year-old children are playing in the yard. The first child says:
"Yesterday I found a condom on the veranda!"
The second child replies:
"What's a veranda?"

The word "veranda" is slightly old-fashioned in US English, and probably 90% of adults -- and nearly 100% of children -- would use the word "porch" instead. A variant of this joke has the first child saying "I found a condom under the davenport" -- davenport being a regional dialect word for "sofa" or "couch."

So, the only translation difficulty is finding a word that a five-year-old Russian child probably wouldn't know, as a replacement for "veranda" or "davenport". Obviously it doesn't have to be a literal translation -- it can be any word referring to a location where someone might have dropped a презерватив.

P.S. I'm not sure how old the English version of the joke is -- I would guess that it goes back to the 1980s, when the AIDS epidemic made it socially acceptable to say the word "condom" on television.