In Wales, when it's raining hard, we say 'Mae'n bwrw hen wragedd â ffyn', which translates to 'It's raining old ladies with sticks'.
In Afrikaans, if you're going to give someone what-for, you tell them 'Ek sal jou wys waar Dawid die wortels begrawe het!' which translates to 'I'll show you where David buried the carrots!'
What are some fun idioms from your language?
@aschmitz Ah yeah, @warpgate9 told me about the Afrikaans one :D