Background speech is annoying and distracting when working on tasks that require focus, and according to previous research, background speech is a common cause of reduced work performance. According to the interference-by-process theory, distraction is a function of the similarity between the processes involved in the involuntary analysis of the background speech and the voluntary processes involved in the task. In view of this theory, a similarity in language—between the produced language and the language that is listened to—may increase the magnitude of distraction by background chatter in comparison with when the language which is produced is different from the language that is perceived. The purpose of the present experiment was to investigate whether work performance—as indexed by writing fluency (WF)—varies depending on the similarity between the language that is heard in the background and the language that is produced. The experiment had a within-participants design with two factors: language to-be-produced (Swedish vs. English) and language of the background speech (Swedish vs. English). The sample constituted of 43 university students, with Swedish as native language. The result showed a main effect of language to-be-produced: WF increased when the participants produced text in their native language compared to text production in their second language. No main effect of language of background speech was found, and no interaction between these two factors was revealed.