Academic self-concept (ASC) is a student's self-perceived level of ability in an academic domain and a major factor involved in academic performance. We examined the importance of internal/external sources of evaluation for the structure of L2 and L3 ASCs in two groups of 5th/6th graders learning English and French in Switzerland (mean age: 12.1). The target languages were introduced to the children in reverse order as part of the Swiss mandatory curriculum: L2 English/L3 French in Group 1 (n = 464) and L2 French/L3 English in Group 2 (n = 157). Network analysis returned a consistent pattern in the constitution of L2/L3 ASCs irrespective of target language and language curriculum. The children hold distinct ASCs for the L2 and L3. Internal affective variables (intrinsic motivation and anxiety) correlate more strongly with L2/L3 ASCs than external variables (perceived encouragement from teachers and parents, comparison with peers). Dedication to FL learning emerges as a crucial construct that mediates the connection between L2/L3 ASCs and the external variables. Cross-linguistic correlations between L2 and L3 are identified for parental encouragement, extrinsic motivation, dedication, and anxiety, suggesting that these are language-integrated dispositions for instructed FL learning. In comparison, intrinsic motivation and perceived teacher motivation are target language specific.