The sentence repetition (SR) test is considered as a promising diagnostic tool for detecting language proficiency in monolingual learners, but less is known about its potential to identify dual language learners' (DLLs) linguistic proficiency. Considering that challenges with language learning, such as developmental language disorders (DLDs), is evident in both first and second language (L1 and L2), it is important to use tools that can assess language functions in both languages whenever feasible. We found that students' L1 and L2 SR performance levels draw upon their phonological short-term memory capacity, vocabulary and grammar skills in the language being assessed. We also found a positive relationship between students' L1 phonological memory and L2 SR scores, suggesting that DLLs' phonological short-term memory capacity assessed in their L1 indicates individual differences in short-term verbal memory that support SR performance, as assessed in L2. The results highlight a challenge in both research and clinical settings which often use monolingual assessment tools and associated norms to identify DLLs who might be at risk of developing DLDs or other learning difficulties.