After changing the locale on a resource to "English India", when that user goes to create or update a date field on a project, they get an error upon saving that says "invalid date format". It appears that even though they changed the user's locale, the data format that is being invoked on the date field selector is MM/DD/YYYY rather than DD/MM/YYYY which is what it should be for "English India".
This is typically caused by a problem with cached form data in the browser.
Have the user completely clear the browser cache, and then retry. Additionally, in Chrome, try using an "incognito window" which does not use the current cache or any add-ons.