This assumption turned out to be wrong, or at least insufficient. This research found that people performed more intelligently and rationally when they were using a second language. They calculated chances better, they were less likely to use heuristics and they solved moral dilemmas in a more utilitarian way.
The reason is that native tongue usage is automatic, while second language needs conscious effort. You need to suppress your automatic reaction of using your native tongue and by doing so, you also suppress heuristics and ingrained mental schemes.
You can test this by trying to say threatening or embarrassing statements to someone (like “I have a small penis so I’ll never find a woman and men will laugh at me”) in your native tongue and in a second language. The second will elicit much less emotions, showing that if the statement was true, you’d be more likely able to rationally deal with it in a second language environment.
This explains something that I didn’t understand. During the medieval times, the few educated people spoke Latin, a dead language that had no practical use. I mean there was no native Latin speakers, everyone had to learn it, why didn’t they use Italian or Spanish or German instead which would save the millions of native speakers and help the others to use the language when dealing with laymen or peasant. In light of this research we can understand that speaking in Latin helped all intelligent people to act intelligently instead of using their emotional/cultural automatisms which were many back then.