It is typically assumed that language and nationalism are strongly related. However, it is unclear whether the acquisition of a “national” language causes national identification. While some argue that the acquisition of a national language leads to the assimilation of individuals into this nation, others argue that language acquisition is endogenous to national identification or that assimilationist pressure might even lead to an identity backlash. In order to investigate the causal relationship between language and nationalism, we analyze language acquisition in the Basque Country where the Basque language expanded very quickly after democratization. We do find that Basque language acquisition does have a substantial positive impact on Basque national identities and even nationalist attitudes. However, this impact is restricted to children of Basque parents and municipalities with a relevant share of Basque speakers. Also, we show that the relationship can only be generalized to other regions with strong minority nationalism. Hence, our study goes beyond the state of the art by demonstrating the strong conditionality of the effect of language on nationalism leading to a reinforcement among those who already identify with the nation. With this finding the article contributes to the literatures on nation building and the integration of migrants.