Dear ksertkaya,
Please, check locale settings for your site. We get similar result in our tests if the culture selected has different decimal symbol. You can even try it yourself, if you replace dot with coma, it's likely to save the value correctly.
While your browser settings are definitely correct as we can see on the attached screenshots, it's SharePoint site settings which are more important here.
You can check it yourself if you go to Site Settings:
- SiteSettings.png (8.41 KiB) Viewed 1993 times
Then go to Regional Settings:
- SiteSettingsRegionalSettings.png (26.71 KiB) Viewed 1993 times
And here you can see the locale that determines number formatting for the site:
- SiteSettingsRegionalSettingsLocale.png (14.39 KiB) Viewed 1993 times
If the locale is set to United States or another culture that also has dot as decimal symbol, then the result is very strange. I recommend you to check a default form to see if it has a dot or a coma as a decimal symbol.
More importantly, I am not sure why the code fails and puts the wrong symbol, but we can solve this issue if we figure out the reason behind saving the incorrect value. If it's culture, you'll either be able to change culture or if it is undesirable, we can write some custom JS for you to replace dot with coma after multiplication.