Children with developmental language disorder (DLD) are at risk for social, emotional and behavioural (SEB) maladjustment throughout development, though it is unclear if poor language proficiency per se can account for this risk, as associations between language and SEB appear more variable among typical language children. This study investigates whether the relationship between language and SEB problems is stronger at very low levels of language, and considers confounders including socio-economic status, sex and non-verbal intelligence. These are examined using a population-based survey design, including children with a wide range of language and cognitive profiles, and assessed using the Strengths and Difficulties Questionnaire and six standardised language measures (n = 363, weighted n = 6,451). Structural equation models adjusted for prior levels of SEB reveal that the relationship of language at age 5-6 years to SEB at 7-9 years was non-linear. Language more strongly predicted all clusters of SEB at disordered language levels relative to typical language levels; with standardised betas of -0.25 versus 0.03 for behavioural, -0.31 versus -0.04 for peer, and 0.27 versus 0.03 for prosocial, problems. Wald tests between these pairs of betas yielded p-values from .049 to .014. Sex moderated the nonlinear association between language and emotional symptoms. These findings indicate a clinical need to support language development in order to mitigate against problems of SEB, and to carefully monitor the mental health needs of children with DLD, particularly in the context of multiple, and potentially sex-specific, risks.