Learners entering higher education may learn specialized vocabulary of new disciplines in their second language (L2) with or without support from their first language (L1). However, these learners cannot rely on an established conceptual representation in L1 when learning L2 specialized vocabulary. The effects of learning a new concept in L1 prior to learning and testing the same concept in L2 are yet unclear. The present study compared the effectiveness of L1-mediation and L2-repetition word learning methods randomly assigned to 75 bilingual adults. Participants learned subject-specific specialized vocabulary through reading a series of definition sentences in two learning episodes a week apart. The L1L2 group (N = 31) first learned L1 words on Day 1 and then their L2 translation equivalents on Day 2, while the L2L2 group (N = 44) learned the words in L2 on both days. In each learning session, participants provided self-ratings on their confidence in acquiring word meaning after reading each sentence and completed a word meaning recognition test afterward. Results using linear mixed-effects modeling showed that L1-mediation learning may ease the start of L2 word learning as shown in higher confidence ratings and less reading time, leading to a faster establishment of high-quality conceptual representations. In contrast, L2-repetition learning may strengthen lexical representations and L2-concept connections through increased exposure to L2 words. However, the two learning methods produced comparable results on L2 meaning recognition, so flexible adoption of the learning methods according to the learning goal is suggested.