There is an effect when speaking a foreign language that I think isn't talked about as much as it should be. The fact that, when speaking in another language, one takes on a slightly different personality than when speaking in their native tongue. I've noticed the effect of this in many different contexts.
I'm a native English speaker, and I've studied many languages to varying levels. But with the German language I got to the highest level and I can understand most conversations. I noticed eventually that I acted differently when speaking German, but I also noticed the converse. I worked with mostly German colleagues, but the working language was English. So my colleagues were used to the fact that usually when they were speaking English it was about work and therefore more serious. Conversely, when at work they would sometimes speak German with each other when a non-German speaker wasn't involved. Since I could understand what they were saying, I routinely noticed that they were more casual with each other. This is surely partly because they can just speak more freely in their native tongue, but I also got the sense that it was psychological, meaning in German they could access a larger range of their personality.
Being now in a multi-lingual marriage, I am interested in how multiple languages effect the raising of children. Without citing anything, I have read that children develop fine with up to three languages, but having four or more at a very young age can slow development. An example scenario with three languages is two parents each speaking to the child in their native tongue, and one more being learned and used at school. It also supposedly helps if one parent always speaks a consistent language with the children. I would guess that one of the reasons that this method is preferred is that the child can consistently develop their intelligence and personality in each language without getting them confused.
One last phenomenon I can propose has to do with language learning for adults. If one has a different personality when speaking different languages, then it also must be that they have to develop this separate personality when learning a new language. I have gotten the sense when watching others try to speak a foreign language that they are apprehensive to take on this new personality, and it hurts their ability to speak well. The most simple way to explain this is putting on a thick French accent when trying to speak French, rather than just saying French words as they would be said in American English. Of course as one would learn French more and more the accent would become more natural, but in the beginning stages the accent is more like playing a role, and I think that some people hesitate to take on that role. The extension for an advanced speaker would be using more phrases and colloquialisms in the foreign language. Sometimes they don't make sense when directly translated, and are a result of different cultural forces, but from just a language point of view they are useful for better communication.
In any case, I think some people would not even agree with the basic premise that when speaking a different language we take on a slightly different personality. But I have seen it and experienced it myself, and I think it actually can explain some aspects of language learning and use.
