In my experience, I can recall some situation when I was a kid, first years at elementary school (at 6 or 7 y/o), when the teacher was surprised because I started writing Spanish (my native language) with English syllabus. For example, "y" (in my country) is pronounced as "sh" in English. I started using that kind of English writing, and remember the teacher was really surprised, because I was a good language student, but she didn't understand why I was doing so.
In my past life (immediately before the current one) I use to be a kid born in 1926 in US, who died in 1944 during WWII in Holland.
Also I can speak and write Italian. I learned the language in just a few months, and I can write it with a very advanced grammar, even if I never "officially" learned Italian grammar in my life. I'm 57 now. I'm working in a US company doing books and magazines typesetting. I normally speak Italian easier than my native Spanish, and some sentences come first to my mind in Italian, even before than in Spanish. My Italian friends are very surprised about the way I can communicate with them in their language.
All this is completely unconscious. I studied other languages. French (with very modest results) and some German (impossible for me to advance with that), but I was a "natural" with Italian, and a pretty fast English learner.