<![CDATA[This study aims to analyze the effect of the intensity of regional language use on the quality of Indonesian language proficiency among ninth-grade students of SMP Islam Al-Mubarok. A quantitative approach was employed using a survey method and simple linear regression analysis to examine the relationship between the intensity of regional language use as the independent variable (X) and Indonesian language proficiency as the dependent variable (Y). The data were collected through research instruments developed based on the study indicators and analyzed using statistical tests. The findings revealed that the working hypothesis (Ha) was accepted, indicating a negative and significant effect of regional language use intensity on students' Indonesian language proficiency. This result was supported by a significance value of 0.000 (p < 0.05) and a regression coefficient of -0.85, demonstrating that a higher intensity of regional language use was associated with lower Indonesian language proficiency. The coefficient of determination (R²) of 61.6% indicates that variations in Indonesian language proficiency are substantially explained by the dominance of regional language use, while the remaining percentage is influenced by other factors beyond the scope of this study. The dominance of regional languages contributes to linguistic interference, particularly in grammatical aspects, and reflects students' difficulties in performing code-switching between informal and formal contexts, potentially reducing the effectiveness of Indonesian as the primary language of academic instruction.]]>
