Unfortunately, Mendix has an implicit configuration for this. Language settings and decimal notations are mapped in a hardcoded fashion, which means whenever a language is decided for a user, decimal notation is also assigned accordingly.
So there is no Mendix built-in solution (known to me at least) to set decimal notation independent of the language settings.
If you have a single language in your project (I assume this language is using dot (.) as a decimal separator which you don’t want), then what you could do is to set each user a different language which is bound to use comma(,) as a decimal separator.
This is the fastest workaround solution for a monolingual project, in which you do not need to change anything else like default language of project or language setting of browsers. Just add a second comma-using language and assign it to be all users language.