1 Introduction

Under the "the Belt and Road" initiative, the challenges faced by China's colleges and universities in cultivating foreign language talents by adopting the "foreign language+foreign language" complex language model are discussed, such as heavy teaching tasks, insufficient learning time, poor learning motivation and effect. It is pointed out that in the current practice of multilingual teaching, universities often independently carry out various language teaching, ignoring the kinship, types, and language contact relationships between languages, and have not formed optimized multilingual teaching plans. Taking "Russian+Kazakh" as an example, this paper analyzes the phenomenon of code duplication between Russian and Kazakh, including overall duplication, selective duplication, and other types, which leads to the convergence of basic code and model code, and significant differences in dialects. Propose the differences between trilingual acquisition and general foreign language acquisition, the impact of second language acquisition on trilingual acquisition, and suggestions for instructional design.

2 Experimental methods

This chapter introduces the method of the "Russian+Kazakh" multilingual teaching experiment. The experiment aims to explore the impact of second language acquisition on third language acquisition, the relationship between first language and third language acquisition, and the role of code duplication in third language acquisition. The experimental subjects were 44 students majoring in "Russian+Kazakh", divided into an experimental group and a control group. The experimental group included analysis and explanation of code duplication phenomenon in Kazakh language teaching, while the control group did not. The teaching process includes explaining key vocabulary, providing picture examples, summarizing and generalizing, and strengthening application. The measurement tool is a sentence meaning judgment ability test, with the same pre-test and post test structures, including questions of different difficulty levels, comprehensively examining students' understanding of code copying components.

3 Experimental results and analysis

The experimental results show that second language acquisition has a significant positive transfer effect on third language acquisition, and there is a significant difference in the accuracy of third language text comprehension between the experimental group and the control group. Both zero start and non-zero start grouping can produce positive transfer effects in second language acquisition. The performance of the zero start group in the experimental group even exceeded that of the non zero start group in the control group, indicating that the positive transfer effect of second language acquisition may exceed that of first language transfer similar to third language.

4 Conclusion

In close language contact, trilingual syntax will undergo significant changes and converge with second language. Teaching experiments have shown that second language acquisition has a positive transfer effect on third language acquisition, which is applicable to different types of code duplication. We should comprehensively carry out multi language teaching, utilize language contact materials, form effective teaching plans, and enhance learners' language abilities. Future research will expand the sample size, explore grammar relationships in depth, and optimize composite foreign language teaching models.