<![CDATA[General Background YouTube provides an expansive digital platform for foreign learners practicing Indonesian. Specific Background Diverse mother tongue backgrounds generate varying phonological errors that risk fossilization without linguistic evaluation. Knowledge Gap Previous phonological studies isolated Asian and European speakers without directly comparing Japanese open-syllable and American closed-syllable frameworks. Aims This qualitative case study describes and compares omission, substitution, and epenthesis errors between Japanese and American speakers while proving mother tongue interference. Results Analyzing YouTube speech revealed the Japanese speaker produced 38 errors dominated by omission and epenthesis, whereas the American speaker generated 29 errors dominated by vowel substitution. The Japanese open-syllable structure systematically triggers vowel additions, whereas the English phonological absence of the independent schwa /ə/ causes vowel substitutions. Furthermore, both speakers consistently omit the word-final nasal phoneme /ŋ/. Novelty This study explicitly contrasts Asian and Western phonological interference mechanisms within digital media speech data. Implications These empirical findings allow language educators to map language-specific pronunciation challenges and design targeted phonological exercises. Highlights The Japanese open-syllable system predominantly triggers omission and epenthesis errors during Indonesian speech. The absence of an independent schwa phoneme in American English primarily causes vowel substitution errors. Both Japanese and American speakers universally struggle with the omission of the word-final nasal phoneme. Keywords Phonological Interference; Vowel Substitution; Epenthesis; Mother Tongue; Fossilization]]>
