In American English a period ( . ) is used as a decimal separator, but there are other places in the world where a comma ( , ) is used.
I would read a quantity of 7.5 as “seven point five” written as-is. But if you use a pause and not a point how do you read the number aloud?
7,5 is read in Finnish as “seitsemän pilkku viisi”. 7.5 would be read as “seitsemän piste viisi”.
The rule is that if it’s in decimal system, you use comma; if in any other system, you use dot. And you read what you write.