I'm not a native speaker but English has been my language of choice for more than 10 years now. I think and dream in English... However, I'm always struggling with numbers.
If I'm watching a video for example and the person says some number greater than 100, I need to slow it down and repeat it a few times before I can internally "visualize" what it actually means. I can perfectly understand sped up videos, but I find it very difficult to understand numbers at a normal speed.
I also find it difficult to say numbers or calculate stuff in my head. My speaking (and thinking) speed drops significantly, and I can even mess up frequently or mispronounce numbers that sound similar (e.g. thirteen vs thirty).
I do try to practice counting and manipulating numbers in my head, but I hardly see any improvements. And this issue also happens with another foreign language that I speak somewhat fluently.
I think that the issue is that different languages have different ways of representing numbers (for example, saying 2 before 3 in 23, or the other way around). And when you learn your first language, the brain structures how it sees numbers based on those rules, and strongly associates the words and symbols with the notion of a "number".
I would highly appreciate any suggestions that might help.