The short vowels were the biggest challenge, because no other major language has to deal with the sounds of "cat in the hat" and "dot com" and "up above". Hebrew in particular is utterly unequipped for it. Vowels in general are the biggest problem. Modern Hebrew, like Spanish, has only five vowels (represented by three letters) whereas English has around 12 to 15.
This is mostly an idle pursuit for my own amusement, but it's not entirely without practical value. If you happen to be a native English speaker who has gone to Israel with a smattering of Hebrew, you know how desperately the Israelis need a consistent way of representing English words in Hebrew. Anyway, it's a work in progress, but there it is.