Another thing that may require extra work is those burned-in subtitles. Here is a quick example of JP with “place-holder” subtitles covered by an area cutout from UK (keep in mind the UK pan&scan shots often could be off-center, as in this demo):

As (not) shown, the UK overlay on JP must have a spot-on color match to cleanly erase the subtitles. Any missed characters would need other approaches, like a de-logo filter (on moving backgrounds) and temporal-frame-overlays from clean frames (on static backgrounds).