Jean-Marc Dewaele of the University of London investigates, for a study titled "‘Christ fucking shit merde!' Language preferences for swearing among maximally proficient multilinguals":
The present study investigates language preferences for swearing among two groups of multilinguals. The first group consisted of 386 adult multilinguals who filled out the Bilingualism and Emotion web based questionnaire (BEQ, Dewaele and Pavlenko, 2001–2003) and had declared that they were maximally proficient in their L1 and L2 and used both languages constantly. The second group consisted of 20 multilinguals with a similar sociobiographical profile who were interviewed about their language choice for the communication of emotion. A statistical analysis of the quantitative data revealed that despite similar levels of self-perceived proficiency and frequency of use in the L1 and L2, the L1 was used significantly more for swearing and L1 swearwords were perceived to have a stronger emotional resonance. An analysis of the quantitative data from the BEQ and the interview data confirmed the findings of the quantitative analysis while adding rich detail about the difficulties in deciding which language to choose for swearing.
In other words, people like to swear in their first language. As one Greek immigrant to Britain put it, "When I say a swearword in English it won't feel as strong as I say it in Greek."
There are some exceptions though. Dewale reports that "Several participants, typically of Arabic or Asian origin reported that swearing in English allows them to escape the social constraint that weighs on them in Arabic, Kurdish and Chinese, where swearing carries strong social stigma."