We investigate how L1 phonology and semantics affect processing of interlingual homographs by manipulating language context before, and auditory input during, a visual experiment in the L2. Three experiments contained German–English homograph primes (gift = German “poison”) in English sentences and was performed by German (L1) learners of English (L2). Both reaction times and event-related brain potentials were measured on targets reflecting the German meaning of the interlingual homograph. In Experiment 1, participants viewed a pre-experiment English film, then half of the participants (n = 16) heard noise and the other half (n = 16) heard German pseudowords during the experiment; in Experiment 2, participants (n = 16) viewed a pre-experiment German film then heard noise; and in Experiment 3, participants (n = 16) viewed the pre-experiment English film then heard real German words. Those who had viewed the English film then heard noise during Experiment 1 showed no L1 influence. Those who saw the English film but heard German pseudowords during Experiment 1, or viewed the German film before and heard noise during Experiment 2, showed L1 influence as indicated by N400 priming of L1-related targets in the first half of the experiment. This suggests that a pre-experiment film in the L1 or the presence of L1 phonology during the experiment slowed down adjustment to the L2 task. In Experiment 3 with real L1 words in the background, N400 priming of L1 meanings was observed throughout the entire experiment for lower-proficiency participants. We discuss our findings in terms of context types that affect L1-to-L2 adjustment.