This paper gives an overview of an Erasmus+ project that aims at empirically identifying the potentials and challenges of language-responsive teaching designs for low-achieving students in vocational contexts by means of topic-specific design research. It delivers first insights into developing and experimenting with two teaching units (percentages and proportional reasoning) and summarizes the questions that come up when adapting design principles for language-responsive mathematics teaching for the teaching and learning of young adults in vocational education. The empirical investigations are carried out with young adult pre-apprentices in lower vocational education in Germany in the technical sector. These young adults are often low achievers in mathematics and often have only limited language proficiency in the language of instruction.