Bilinguals usually select the right language to speak for the particular context they are in, but sometimes the nontarget language intrudes. Despite a large body of research into language selection and language control, it remains unclear where intrusion errors originate from. These errors may be due to incorrect selection of the nontarget language at the conceptual level, or be a consequence of erroneous word selection (despite correct language selection) at the lexical level. We examined the former possibility in two language switching experiments using a manipulation that supposedly affects language selection on the conceptual level, namely whether the conversational language context was associated with the target language (congruent) or with the alternative language (incongruent) on a trial. Both experiments showed that language intrusion errors occurred more often in incongruent than in congruent contexts, providing converging evidence that language selection during concept preparation is one driving force behind language intrusion.

In bilinguals, language proficiency has been advanced to influence the involvement of domain-general control networks in language selection. We assessed, in university student translators with moderate- to high-second language (L2) proficiency depending on their translation educational level, the functional activity in the key language and control areas (the caudate nucleus, anterior cingulate, and prefrontal cortex), during task- and language-selection in an oral production context. We found that L2 proficiency influenced the relative involvement of our regions of interest during language selection vs domain-general cognitive control processes. While the left middle frontal and left caudate areas were more involved during linguistic than alphanumeric task selection in the low L2 proficiency group, these regions were similarly involved in both tasks in the high-L2 proficiency group. These findings suggest that language selection relies primarily on a network within domain-general cognitive control system with an increase in resource needs when L2 proficiency is low.