A fundamental question in developmental science is how brains with and without language compute numbers. Measuring young children's verbal reactions in Spain and Finland, we show that, although there is a general arithmetic ability for small numbers that is shared by monkeys and preverbal infants, the development of such initial knowledge in humans follows specific performance patterns, depending on what language the children speak (here, Spanish and Finnish). Together with our previous data collected in France and in England, these new results contribute to a European cross-linguistic cartography of the relationships between language and number development.