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
I believe linguistic relativity and Sapir-Whorf Hypothesis are the usual names for this idea. I also believe it's been mostly discredited in linguistic circles, but it's been a while since I talked about it with a linguist, so you should do your own research.
Thank you! I tend to remember ideas better than their names. (And linguistic relativity is a horrible name for that idea, IMHO.)
A quick glance around the internet suggests that Sapir and Whorf's versions of the theory - that language affects thought, but doesn't strictly determine it - enjoys moderate empirical support and continuing professional support. The stronger variants, broadly linguistic determinism, seem to be largely discredited.