I've been looking for a way to get English party chat on DQIV DS and referenced info from this thread (among other places) to put together a tool that patches the English text files from the US ROM and Android app so that they are formatted correctly (or at least acceptably) for use with the Japan ROM, to avoid having to manually fix all the files in a hex editor. I wanted to share this tool in case someone finds it useful, but since I don't have any other posts I can't post a link here - you can find the link as the homepage in my profile (does not include any ROMs or Android files, you must supply them).
This tool requires manual steps to unpack the US ROM using dslazy to pull data, extract the Android OBB file to get the party chat files, and re-pack the Japan ROM using dslazy, which is all documented in the link. I may look at automating that in the future.
If have questions on how to use the tool or you want to report issues/make changes to the patch, let me know here or on Github. Hope someone finds this useful!