The u in juli is a "longer" ponunciation, check here to compare: http://www.heardutchhere.net/DutchPronunciation.html#J
Kind of. You could say "Is jouw verjaardag in augustus?", which translates to "Is your birthday in August?". Jullie is the plural "you" (think of the southern American y'all), so this sentence refers to multiple speakers; "Are y'all's birthdays in August?" or more commonly "Are your birthdays in August?".