For learners at low levels using L1 is recommended as L2 definitions will probably comprise more unfamiliar words making it difficult to comprehend and require retaining too much new information.
For higher level learners L1 translations provide easier and faster opportunity when you aim at learning just particular words. If your aim is to improve overall language level L2 definitions are good option. Because they will make you learn more phrases and more words in the long run. Notice that L2 definitions are more time consuming.
And of course, combining two strategies may give you benefits of both methods.
L2 vocabulary learning and testing: the use of L1 translation versus L2
results showed that the students’ recognition of the L2 vocabulary was significantly
higher when asked to match the target vocabulary to L1 translations than L2
definitions. Furthermore, there was a significant interaction between study language
and testing language, such that when study and testing language matched, the
participants scored significantly higher. However, overall, the language in which the
target vocabulary was studied did not matter for test scores.
Using L1 in Teaching Vocabulary to Low English Proficiency Level
Students: A Case Study at the National University of Laos
Using 169 students of a low proficiency level, it
was found that using learners’ mother tongue (L1) to teach English as a foreign language in Laos enhanced their
retention of new vocabulary items both in isolation and in context. This is possibly due to clear definitions and
explanations in L1, dictation quiz and translation exercises in the classroom.
The Effect of L1 on Learning New L2
Vocabulary among Intermediate Proficiency
The overall findings of the study
indicated that there was no significant difference between the comprehension of those who received Persian
translation of new vocabulary and those who dealt with the paraphrase of the same new words and those who
did not receive any L1 translation or paraphrase. The results for vocabulary test showed those who received
the mother tongue definitions in front of each new word in the passage did perform significantly better on the
vocabulary test than the other two groups. However study would help teachers and teacher trainers to
construct and implement L1 and paraphrase in English classes more effectively.