Date format for localized sites should use national standards everywhere, even in tooltips for reputation graph. Currently SO in Russian uses same format as English SO:
In other words,
day-of-a-week, month day, year. I suggest to swap
month, like it is used in the system clock tooltip in MS Windows by default:
Worth noting that format in table mode is already correct:
Adding a string with date format into transifex will allow fixing the issue.