An idiomatic way to say a task is pointless is to say it’s like carrying coals to Newcastle – Newcastle, in the north east of England, used to be a major coal mining area.
In French the equivalent is porter de l’eau à la rivière (to carry water to the river).
In German they say Eulen nach Athen bringen/tragen (to take/bring owls to Athens).
In Welsh there are quite a few equivalent expressions:
– cario glo i Fflint = to carry coal to Flint (a former coal mining area)
– cario dŵr dros afon = to carry water across a river
– bwrw heli yn y môr = to throw salt in the sea
– iro blonegen = to grease fat
– iro hwch â bloneg = to grease a sow with fat
– gwerthu mêl i berchen gwenyn = to sell honey to a bee keeper
– mynd i ‘ngheg i chwilio am fy nhafod = to go to my mouth and look for my tongue
– gyrru halen i’r Heledd = to send salt to a salt pit
– golchi traed alarch = to wash a swan’s feet
– taflu ‘fale i’r berllan = to throw apples into an orchard
What about in other languages?