One can daven in Lashon HaKodesh even if he does not understand what he is saying. If one chooses to daven in another language, he must understand what he is saying.
Technically, Lashon HaKodesh is NOT modern day Hebrew. While many words are identical, many have been changed. The old joke about a seminary girl (of course!) that gets on a bus and asks the driver ??? ???? ? He responds with ??? ????? !!