Originally Posted by
atom0s
The servers don't care what language you connect with, they'll give you updates either way. The updates are the same for every language client. All the files downloaded are the same for English and Japanese. If you want to type in Japanese, you can just use things like Ashita's IME addon which enables the Japanese IME on non-Japanese clients. Windower had an IME plugin at one point but I have no idea if it's still maintained.
As for editing DATs directly, it depends on which ones you plan to edit. If its simple mob renaming, those files are in a format that allows for plain text editing as long as you are careful not to edit certain bytes or go too long on a name string. Other dat files are encoded, bit packed, etc. as Cair mentioned. Which requires decoding them, editing in a proper manner, then reencoding them properly. There are not really any tools available currently to edit files since it's not really seen as a major thing to do on FFXI outside of modding models and stuff, things like editing events, strings, etc. are usually not done here since updates will override any edits you make and it is more or less a hassle to keep redoing them.