Some years back now, I had a mini stroke during which I lost completely the ability to write my mother tongue or rather to type my mother tongue which is English. Whilst in the throes of this stroke, I would write a word like “difficult” with four or five ‘i’s and half a dozen ‘f’s then stare at it knowing that wasn’t right but being unable to work out what was wrong with the way I had spelled it!
The neurologist that I eventually saw said that I'd had a transitory ischemic attack in the area of my brain that deals with my mother tongue to stop I asked him whether, had I been typing in French, my second language, if I would have been similarly impacted. He said no apparently one's mother tongue functionality resides in one area of the brain and any other languages that one learns after the first one forms a new language area in another part of the brain altogether. This is one of the advantages of learning languages other than one's mother tongue; it opens up pathways and areas in the brain that would not otherwise be present. This is also allegedly why every language after the first foreign language one learns is easier to learn because the foreign language learning department of the brain and it's necessary links to the rest of your brain have already been installed, as it were