It could be for a number of reasons - because we want to emphasize on the emotional value of the word or expression(e.g. the irony of a British ''really'' or ''interesting''); because we are in the mood to speak that language, feeling some connection to it or being connected to it via job/ daily routine (e.g. if you read or interact in a foreign language most of the time, then it becomes a natural interface; or if we relate to it by past experience); or because there are unique words and phrases in a language which don't exist in other languages -- we want to describe a particular situation with precision.
The first appealing fact is indeed that people do study, plunge into, and live other languages. We should not take this for granted. Such an experience entails broadening the experiences offered already by ther mother language, but also enlarging the ways not ony how we can express ourselves, but also how the world comes to us precisely in the general form of another cultural experience: learning, practicing, and living a foreign language.
As we all know, each language has its own dimension for seeing and experiencing the world, namely others, nature, time, etc. It is only after some people have learnt other languagaes that, yes, they can use expressions, words, nuances and the like in a foreign language. In a way it all depends on the force or the initial strong impression a word or an expression in a foreign language had that we can bend over that way, rather than in our mother language.
Above all, language is a communication tool, so I'd say that the language(s) you use is/are very dependent on the person you address. Between 2 persons, it is not so easy to switch languages, once you get used to such an unwritten agreement, even when - at a certain moment - you're both more proficient in a second language. A reason to express something in a different language can be that it is just much shorter or simpler in the other language.
I have seen parents teaching their children a different language than their mother tongue to show to their peers that they are more advanced , successful , posh etc , result being children learn mixture of things and a stage comes when they even get confused what they are talking about . But I suppose in long run it does work and ultimately they do master both the languages , though may not learn to best of them.
Thank you so much guys! all the feedback are very appreciated.
Sigmund Freud told me that the reason people tend to use other languages than the mother tongue is to ensure their inner expectations. I don't like the idea as I use foreign languages to swear a lot. arrrrrrrrrggggggghhhhh.
I find it interesting that Sigmund Freud spoke to you.!? But I would like to add that the nature of the topics being discussed may be a determinant of the languages spoken. For example, I have received my education in different languages, and I would speak a language depending on who I am with and whether the topic being discussed is easier for me to engage with in either English or another language.
In India some universities require studying a foreign language such as French, German or Russian with the idea that there is substantial research literature in those languages and the researcher would be benefited in reading them in the native language for his/her research.