The researchers aren't entirely sure why speaking in a less familiar tongue makes people more "rational", in the sense of not being affected by framing effects or loss aversion. But they think it may have to do with creating psychological distance, encouraging systematic rather than automatic thinking, and with reducing the emotional impact of decisions. This would certainly fit with past research that's shown the emotional impact of swear words, expressions of love and adverts is diminished when they're presented in a less familiar language.

Paywalled article (can someone with access throw a PDF up on dropbox or something?): http://pss.sagepub.com/content/early/2012/04/18/0956797611432178

Blog summary: http://bps-research-digest.blogspot.co.uk/2012/06/we-think-more-rationally-in-foreign.html