Vocabulary experts recommend first language (L1) translation equivalents for establishing form-meaning mappings for new second language (L2) words, especially for lower proficiency learners. Empirical evidence to date speaks in favor of L1 translation equivalents over L2 meaning definitions, but most studies have investigated bi- rather than multilingual learners. In our study, we investigated instructed English vocabulary learning through an intervention study in six language-diverse secondary school English classrooms in Sweden (N = 74) involving three conditions for presentation of word meanings: (a) definitions in the L2 (English), (b) translation equivalents in the shared school and majority language (Swedish), and (c) translation equivalents in the shared school and majority language plus other prior languages among the learners (Swedish and other). Based on overall weighted mean effect sizes and mixed-effects modeling, the results showed that conditions that involved L1 translation equivalents yielded higher scores than did target language definitions in immediate posttests with a small effect size but no differences in delayed posttests.