Young learners' language aptitude is understudied due to a lack of tests covering this period of life. Young learners, in contrast with adults, are still acquiring their L1. Consequently, a language-dependent aptitude test should be carefully designed for it to be valid. An additional challenge is found when the young learners who are to take the test are bilingual, as is the case of the Catalan/Spanish community in Catalonia. This chapter aims at explaining the process to translate and adapt the Modern Language Aptitude Test -- Elementary in Spanish (MLAT-ES - Stansfield et al., 2005) into Catalan (MLAT-EC -- Suárez, 2010) so as to obtain a valid aptitude measure across grades. The results show that despite the linguistic proximity between Catalan and Spanish, several issues had to be considered, not only linguistic and cultural but also the learners' age range for the test is addressed to.