In the context of language processing, proficiency and age of acquisition have reliably been shown to have a strong influence on the functional and structural architecture of the human brain. The aim of the present EEG study was to examine the impact of language training as experienced by simultaneous interpreters (SI) on auditory word processing and to disentangle its influence from that of proficiency and age of acquisition. Eleven native German SI and controls matched in L2 proficiency and age of acquisition were asked to judge whether auditory presented disyllabic noun pairs both within and across the German (L1) and English (L2) languages were either semantically congruent or incongruent. We revealed enlarged N400 responses in SI while they detected incongruent trials both within the native (L1) and non-native (L2) language and also while they performed the task in the opposite direction as specifically trained (L1 to L2). These enlarged N400 responses in SI suggest a training-induced altered sensitivity to semantic processing within and across L1 and L2. The enlarged N400 responses we revealed in SI to congruent noun pairs during the German-English condition (L1 to L2) may indicate that SI could not benefit from an L1 prime when the target was a L2 word, suggesting additional processing resulting from long-term backwards (L2 to L1) training.