I've been working on the ._dt files in the text directory. Some of them have the same exact content so I was able to simply copy the English version over, but others have new content or ran off the side so I wrote patches for those. So far, these files are working in English:
t_book03._dt
t_book04._dt
t_book05._dt
t_book06._dt
t_book07._dt
t_cook2._dt -- added extra newlines to fix lines running off the side
t_memo._dt -- several new strings added to Evo; translated these by hand
t_name._dt
t_quest._dt -- it turns out FC Evo didn't add any new quests -- both versions had 86 slots for quests. Maybe new quests were only added in the other Evo games.
t_shop._dt
t_town._dt
t_world2._dt
And there's still work to be done on these files:
t_book01._dt -- need to fix image positions
t_book02._dt -- need to fix image positions
t_item2._dt -- 37 new items added to Evo, need to be translated by hand or referenced from later Trails games such as Cold Steel
t_magic._dt -- 21 new magic arts added to Evo, need to be translated by hand or referenced from later Trails games such as Cold Steel
t_monsnote._dt -- still needs to be reverse engineered, and I know new monsters were added to Evo so the English version can't be used directly
I was also looking at the things that remain in Japanese even after all these _dt files are patched. The settings menu is still in Japanese, as well as various system text, not surprisingly. There are a few other miscellaneous things too, like quest statuses ("Finished", "Expired", "In Progress") and shop types ("Arms & Guards", "Lodgings", "Septian Church") that appear over the shop name. And also several shop/inn interactions as well.
All of these strings are in the eboot.bin, when viewed with utf8 encoding. This part will probably be difficult or near impossible for now -- I assume vita eboot hacking hasn't progressed very far at the moment? I don't know much about this topic. I was able to hex edit some English translations in (but of course, I fit the English text into the length of the original Japanese text so that the overall eboot.bin length was unchanged). That actually did work, but still, I probably can't replace the Japanese strings with longer English strings unless the whole vita eboot.bin is reverse engineered, so this is a bit of a roadblock. I suppose in the worst case I will attempt to translate to English just via hex editing and keeping the English strings short, but it will be ugly. I will probably focus on finishing the remaining _dt files before I look at this again.