As I understand locale is being set after user logins. Mendix looks for default locale defined in account settings. Is it possible to choose locale in login page and use chosen one, but not defined in account settings. If it is possible, how can I do it, set cookie via js/pass as a get/post parameter?
You could make a custom login page that let users choose their locale before they login. That locale could then be saved to the user profile so that a user wouldn't have to select locale every time they login. This would be a pretty quick exercise using a custom login page and a simple microflow to store the chosen locale.