Vocabulary acquisition remains a persistent challenge for many EFL learners, particularly in contexts where

exposure to English outside the classroom is limited. In Indonesian classrooms, learners often rely on their first

language as a bridge when encountering lexical difficulties, yet teachers frequently discourage this practice of

code-switching. This study employs Classroom Action Research (CAR) to investigate how code-switching can

be used as a cognitive strategy to enhance vocabulary learning among Indonesian-English bilingual senior high

vocabulary acquisition and foster positive learner attitudes toward bilingual strategies. The study not only

addresses a practical classroom challenge but also contributes to a broader pedagogical shift by reframing

code-switching from a sign of weakness into a purposeful and effective learning tool. In doing so, it seeks to

provide teachers with actionable strategies for improving vocabulary instruction in bilingual EFL contexts.

English as a Foreign Language (EFL) instruction in Indonesia continually faces the challenge of fostering

effective language acquisition, especially in the domain of vocabulary. Vocabulary is the foundation of

language proficiency; without a sufficient lexicon, learners cannot comprehend or express themselves

meaningfully in the target language (Schmitt, 2010). Consequently, many Indonesian senior high school EFL

senior high school students are inherently bilingual, navigating both Bahasa Indonesia and English. Ignoring

this linguistic reality can be counterproductive.

viewed with skepticism, often seen as a sign of linguistic deficiency rather than a strategic resource (Gumperz,

1982). However, contemporary perspectives argue that when used strategically, code-switching can facilitate

understanding, manage classroom interactions, and scaffold complex concepts (Ferguson, 2003). For

vocabulary acquisition, it can bridge the gap between known L1 concepts and unknown L2 equivalents,

reducing cognitive load and enhancing comprehension.

This research is important because it challenges the long-standing pedagogical assumption about L1 use in

EFL classrooms. By investigating code-switching as a facilitative strategy, this study aims to provide empirical

bilingual senior high school EFL learners?

use of code-switching as a strategy for vocabulary acquisition?

acquisition in an EFL senior high school classroom?

Vocabulary is widely considered the foundation of language proficiency (Schmitt, 2010), and a lack thereof

can lead to learner frustration and demotivation (Thornbury, 2002). Traditional EFL instruction has often

promoted a strict monolingual approach, forbidding L1 use to prevent "interference" (Cook, 2001). However,

this view neglects the cognitive assets of bilingual learners. More recent scholarship posits that a learner's L1

previous research has examined code-switching for classroom management (Sert, 2005), its systematic use as a

planned pedagogical tool for vocabulary acquisition remains underexplored, particularly in the Indonesian

senior high school context. This study aims to fill that gap by investigating its purposeful implementation and

impact.

Research Design: This study employed a Classroom Action Research (CAR) methodology, which is ideal for

investigating and improving educational practices within a natural classroom setting (Kemmis & McTaggart,

2005). The research was conducted in two iterative cycles, each following the plan-act-observe-reflect model.

This cyclical approach allowed the researcher, who was also the teacher, to systematically implement

strategies, monitor their effects, and refine the intervention based on observed outcomes.

vocabulary acquisition. The scores were analyzed using descriptive statistics and paired sample t-tests

to determine statistical significance.

 Qualitative Data: Data were collected through classroom observations, student reflection logs, teacher

diaries, and semi-structured interviews. This data provided rich insights into student perceptions,

thematic analysis (Braun & Clarke, 2006) to identify, analyze, and report patterns and themes.

Findings from both quantitative and qualitative data sources were triangulated to enhance the credibility and

The research was conducted across two cycles, and the data revealed significant positive outcomes.

Vocabulary Test Results: Quantitative data showed consistent improvement in students' vocabulary mastery.

 Cycle 1: The use of direct L1 translations and bilingual glossaries resulted in an 18% improvement in

test scores. However, observations noted that some students became dependent on literal translation.

 Cycle 2: After strategies were refined to include more contextual L1 explanations and encourage

 In Cycle 2, student-initiated code-switching emerged as learners used Bahasa Indonesia among peers to

clarify meaning and check comprehension. This led to increased engagement, participation, and more

dynamic collaborative work.

Student Perceptions: Student reflection logs and interviews indicated overwhelmingly positive attitudes.

 Most students reported that code-switching reduced the stress and anxiety associated with learning

difficult vocabulary.

 They found L1 explanations helpful for connecting new English words to familiar concepts.

 While a few students worried about "relying too much" on L1 in Cycle 1, they noted that the refined

strategies in Cycle 2 pushed them toward greater responsibility and autonomy.

using L1 lowered anxiety and increased motivation, creating a more relaxed learning environment conducive

Crucially, the study demonstrated a shift toward greater learner autonomy. In Cycle 2, students moved from

being passive recipients to active agents who used L1 strategically to support their own learning and that of

their peers. This reflects the idea that code-switching can support interactional competence (Lin, 2008) and that

students can learn to regulate their L1 use as a learning tool (Macaro, 2005).

Finally, the results highlight the necessity of purposeful implementation. Unstructured or excessive code-

switching risks creating dependency, as seen in Cycle 1. However, when planned and refined, it empowers

learners. This reinforces the notion that teachers must consciously plan when and how to switch codes to serve

specific instructional goals (Sert, 2005).

learner autonomy rather than dependence.

Pedagogically, this study suggests that EFL teachers in similar contexts should reconsider rigid monolingual

practices and instead explore structured bilingual strategies to support their students. Future research could