I grew up in a only German-speaking region, I grew up in a household only speaking German, I still live there, I talk German with my friends, only ever learned English at school, and yet one day I started thinking almost exclusively in English. At some point I started watching movies, shows and Youtube only in English, which might be one of the main reasons this happened.
It´s like my native language sort of switched (that is an overstatement, but you get the idea), I sometimes have to actively translate a sentence I thought in English into German to talk to someone.
There are a lot of Anglicisms the people around me use, which I find painful to listen to, but still do myself. But more likely I will just completely switch to English in the middle of a sentence for no reason until I reach a point where I only have a fitting word in German, from where I will switch completely back. Especially when I´m talking about a piece of media or so that I only read/watched/played in English, I find it incredibly hard to explain it to someone in German. Me and my brother both do the switching thing, so we usually talk in a wild mixture of English and German with each other. Our parents do understand us, but find it very irritating when we do it, so when what we are saying is directed at them, we will usually stick to German.
On the topic of translating, I write stories in both English and German, but I can´t really translate a story I wrote in one language into the other, because I would use different words to convey the same feeling. It works if I think about it for a while, but only slowly. However, when I translate the French Tintin-comics we have lying around to my siblings, I have almost no problem. But with comics the visuals already communicate a lot of the feeling plus I probably wouldn´t even understand the connotations in French.
Not just with English and German, but I also often think a sentence in one language and start talking in a different one, which a) confuses myself when I suddenly run out of words, and b) annoys everyone around me.
When I was still learning French at school it happened to me in both English and French classtests that I would translate words into the wrong language and not notice at all. Otherwise I don´t really mix languages up when writing.
But generally I start switching languages whenever I have the opportunity. I dream in a wild mixture of every language I know even a few words of, when I´m on vacation in France I will start speaking French even with my non-French-speaking siblings, when I´m in Italy I will at some point start speaking broken Italian. As of late I´ve also been known to text people in Finnish despite knowing that they don´t understand a single word. In my family mainly to annoy them, but I also caught myself already having typed "Goodbye" in Finnish in a message to a (non-Finnish-speaking!) friend who doesn´t even know I´m learning Finnish before.
But yeah, while I find quick language-switching relatively easy and often do it unconciously, I know it is painful to listen to and not exactly good for keeping up conversations.
it is magical, isn't it!! i find it so cool that humans have such a large variety of ways to communicate with other humans, and also the creativity that goes into different grammar structures, even if a native speaker of x or y language doesn't see that grammar structure as interesting as a new speaker might. I also love those moments where you're learning a new language and you, i dunno, listen to a song or try to read a sentence, and realize, "hey!! i understood some of that!!". Those moments of excitement and joy are largely why i like learning new languages 
Also this ^^
