Script porting/backporting has happened a few times over the course of ROM hacking's history, indeed aiming for an uncensored/complete work is usually the driving reason for doing such a hack. The biggest technical* issue that takes proper time, effort and skills to sort are when the English script/font/text engine has additional options for letters with tails, variable width fonts, a different encoding and so forth that you might need to work around -- it is not all that common in translated games (though the PS1 was the start of things changing here) and amusingly most people translating from Japanese to something else will tend to want to add this but it does happen.
Otherwise yeah it is usually just standard script extraction, editing and insertion. I guess you might have to tweak the Japanese font a bit if it does not have a good Roman character set but that is usually quite doable.
*the biggest non technical issue is making sure the story, terms and tone all match up.
Edit. Forgot to say this was mainly looking at different regions on the same system, one system to another (even something as otherwise closely related as the GBA and DS) has frequently been seen to cause all sorts of traumas.