Might also be a context switching thing
Like, when I have a dedicated space to go for work, then I find that really helps me to get into the right headspace. My productivity has always been shit when I've lived somewhere that doesn't have enough space to do this.
Maybe what's happening is that the different language forces you to be in a different headspace, which for some reason, helps you to focus better.
This theory is weakened somewhat by the fact that your mother tongue is Portuguese, and you don't find your focus to be improved by English.
It does feel intuitively plausible to me that there is some underlying linguistic thing going on here. There might be some research studying the link between different languages and ADHD experiences, because it does seem like there's something interesting there. If there isn't currently any such research, I have no doubt that it's just because it hasn't been done yet (the wide domain of "academic research on autism and/or ADHD that respects the personhood of the people being studied" is unfortunately, a relatively recent development, but I have been pleased to see that it has been growing rapidly in recent years). If I find anything, I'll report back (which may be in many weeks or months)