I swear a lot in all my languages. I sweared a lot less in French, because I was trying not to make a worse impression than necessary during my Erasmus
But I swear a lot. I try hard to stay within the rules of this forum, with my log being the most tempting place for mistakes in this area.
By the way, have you also heard the myth, that "there is only one swear word in French. Merde. Or perhaps some derived words, but nothing else"?
It is part of the "oh, it is such a romantic language, language of high culture and classical literature, everything sounds like poetry in French" idea. I heard that myth many times. And the people didn't believe me the opposite, even when I told them examples, and when they didn't know French themselves!
If only they knew how rich the French language was!
And I learnt some nice Spanish swearwords in a lab. Cutting stuff on a microtome makes people rememeber those a lot
The essay was really good.
I am thinking in any of my languages, despite not being bilingual, and sometimes totally randomly. Sometimes, the moment is simply made for a thought in Italian. Sometimes I catch myself in an intensive inner discussion (like those moments you surprisingly come up with the perfect answer to some nonsense someone told you a few days before) which just happens to be in Spanish. There is no rule. But of course my inner Italian is bound to be worse than my inner French or English.
The author talks about borrowing words from each of the languages. It happens here on the forums too, or in our real lives. I guess everyone has sometimes just thought (or even used) a saying, that was not originally in that langauge, and was not always understood. Or invented a word based on a foreign one. But her example of the family Franish version was absolutely awesome. Perhaps that is how Catalan was invented, ages ago.
It is sometimes even funny, when some things are easier done in one language, than the other. For example, it is extremely hard for me to write longer texts in Czech. The thoughts simply don't come so easy, the sentences refuse to just get pinned on the paper or display. My Czech grammar is good, but the style is much worse. It is also easier to talk in French than in English. And it had been so for a long time, long before Erasmus, long before C2.
Do you think bilingual writers white their whole books in one language? Even the first draft? Or do they sometimes need to translate a chapter afterwards?
, I can swear that I've never used swear words, either in English or in Russian, on this forum. 
, than when I write in my native tongue. Speaking more seriously, the reason is that in Russian I can write more carelessly (let alone the grammar). I know how those words work, what will be seen as a joke and what as an insult. I know when and where I may cross the line or step on the line. I can make the choices. With English it's like a walk across a mine field. So, I think, that "rule" of kind of distancing yourself from foreign words works when you are C1/C2 already. 
