Numbers cell format adopts local region accountancy formats, not international

You have a choice of using either the comma or the point for the decimal separator. There is no mixing of the two. In your first screenshot, you can see 1234.00 is aligned on the left of the cell. It is text. 1234.00 is not a number in regions where the decimal separator is a comma.

You can change the region of the spreadsheet to one that uses the decimal point (like United States). File->Advanced->Language&Region. Choose a language then choose a region. Any numbers in your spreadsheet will be converted to the new settings. 0,00 will become 0.00. If you have any that were being treated as text, such as 1234.00, you may have to re-enter them to get them to become numbers. Formulas will convert to using a comma, not a semicolon, between the parameters and any new formulas you write will have to be written using commas between parameters.

Can you post a screenshot of what you are seeing?

By default it should be System - yourchosenlanguage.

Click on that and set the language then you can click in the lower box and set the region.

Click on the first box and change it from "System-whatever" to a specific language. Then the second box will be selectable.