I learned French at school (but have forgotton it) and learned the French numeral systems the same way I learned my native language, too. I mean I learned that 90 is 20 times 4 plus 10 and 91 is 20times 4 plus 11 - as mamaDoudou said - but actually I memorized the 1 word itself rather than the mathematical operation behind the number
.
In Vietnamese 43 is four tens three (or shorter four-four). I find in English it's the same (at least for me): fourTY four.
Well, about German numbers... German is second - almost - native languages, too. I went to German school from grade 2 on. But I still find the German numbers terrible. I often get confused about German numbers. When they say 25 (5 and 20 in German) I need to rethink if they mean 25 or 52. There was a funny story about that number. I asked a hotel for my friend and they told me that the room would costs 52 EUR but I "understood" 25 EUR on the telefon. Fortunately, my friend stayed there only for one night!
More over, the German numbers are long to speak 25 is "fuenfundzwanig". Vietnamese in comparison: hai nam or English twenty-two is still shorter. I think speed is important when you do math. I have a Chinese friend. She told me one reason she do math in Chinese is because the German numbers a too long to speak (or think it out).
However, for me numbers are numbers. When I do math I see the numbers as the
mathematic language itself and not German, not Vietnamese etc. But my experiences are limited since I am only familiar with 10base systems.