“Go off on a tangent”in other languages

Same idea, different pictures

English borrows from geometry: a tangent touches a circle at one point and heads off on its own. German jumps from the hundredth to the thousandth, vom Hundertsten ins Tausendste, losing track along the way.

Spanish wanders off into the hills of Úbeda, irse por los cerros de Úbeda. A popular story tells of a knight who avoided a battle there and later blamed the hills for getting lost.