Learner's first language (L1) could influence the learning process of a second (L2) language across the different linguistic levels. The pronunciation of second languages usually accented by the first language phonetic system.  This accented speech impacts learners' expressive, and receptive skills to different extents (Derwing and Munro 2005)[i].  Many researchers mentioned that the evaluation of the process of learning L2 shouldn't rely neither  on the amount of the learned vocabulary nor on the grammar. Otherwise, they focused on the importance of acquiring an accurate pronunciation for a mutual intelligible communication between non-natives and naitive speakers.  (Derwing & Munro, 2015;)[ii]  (Levis, 2018 [iii]).  Accurate pronunciation of L2 governs by many factors such as  the extent of the linguistic difference and similarity between L1 and L2. This extent  could subsequently influence the type and degree of L1 transfer into L2 pronunciation.  Transfer,  could be either negative or positive.  In the case of lacking the L1 phonetic and phonological system to some  features of L2, the interference of L1 into L2 could be regarded as negative transfer. These dissimilarities my cause perception and production challenges to the L2 learners.
[i] Derwing and Munro 2005 Accent and Pronunciation Teaching: A Research Based Approach TESOL Quarterly Journal Volume39, Issue3Pages 379-397   [ii] TRACEY M.Derwing & MURRAY J. MUNRO (2015) pronunciation Fundamentals: Evidence-based Perspectives for L2 Teaching and Research (Language Learning & Language Teaching) John Benjamins Publishing Company; UK ed. Edition.    [iii] John M. Levis (2018),   intelligibility, Oral communication, and the teaching of     Pronunciation, Cambridge University Press