<b>Data-</b><b>The </b><b>E</b><b>ffectiveness of</b><b>T</b><b>ranslanguaging </b><b>F</b><b>orum </b><b>D</b><b>iscussion on EFL </b><b>U</b><b>ndergraduates</b><b>’</b><b> </b><b>L2 </b><b>W</b><b>riting </b><b>P</b><b>erformance: </b><b>a</b><b> </b><b>M</b><b>ixed-</b><b>M</b><b>ethods </b><b>S</b><b>tudy</b>

This study investigated the efficacy of translanguaging forum discussion on L2 writing performance of EFL undergraduates through a mixed-methods approach over an academic semester, comprising quantitative quasi-experiments and qualitative reflective diaries. A total of 155 participants were recruited from a comprehensive university in southeast China and randomly divided into three groups: the experimental group (n = 50) participated in translanguaging forum discussion, control group 1 (n = 53) utilized English-only forum discussion, and control group 2 (n = 50) did not participate in forum discussion. Pre-, post- and delayed post-tests of L2 writing proficiency were administered on the WEWrite platform to assess the participants' progress. The outcomes indicated that all three groups demonstrated enhancements in L2 writing. The English-only participants made the quickest improvements, while the translingual participants displayed more sustained improvements. Additionally, the reflective diary indicated a high degree of learner satisfaction in the experimental group than control group 1, who expressed needs for language support when using the English-only forum; The translingual forum appeared to increase self-efficacy in learning, improved critical thinking, expression, and comprehension, and enhanced communication and socialization better than the English-only forum. Drawing on the theory of psychological ownership, the study offers insights into the application of translanguaging in online learning to enhance EFL learners' writing performance.