This is not possible in a number field. You can use a formula field to replace the dot with a comma and do other formatting. However, the resulting formula field will be for display purposes only and will not affect the field used for input.
And if it’s important for you to have calendars & date field dropdown menus that start on Mondays instead of Sundays, please let them know that too.
Unfortunately, Airtable’s team is entirely USA-based and the product was unfortunately designed for usage in USA & Canada only, which I think is a gigantic problem with the platform.
The more that Airtable hears from people like yourself about this problem, the greater chance we have of them waking up to their blunder and fixing it.