Early mathematical skills contribute to later school performance and socio-economic status. Working memory is related to mathematical skills, but spatial and language skills have independent effects on separate areas of mathematical skills such as solving word problems or arithmetic skills. In this study, 9- to 10-year-old children's (N = 57) cognitive and mathematical skills were assessed three times over 2 years. Spatial reasoning was associated with performance in solving word problems and working memory with performance in the arithmetic test. Regarding word problems, girls outperformed boys. Also, the second-language learners performed similarly to the native speakers in the beginning and in the end of the third grade, but worse in the end of the fourth grade. The results suggest that the students' inadequate mastery of the school language may lead to underachievement in mathematics, implying that further efforts should be invested in supporting their language learning.