For a complete and correct localization, you will need to replace the image base address and resources of about 100 files with those of your language version. - https://win2k.org/blog/2019/10/07/1989811/
That means localization requires collecting files from various updates such as KB891861, XP SP3, SV2003 SP2, non-public updates, etc. I'm tracking down what I actually need.
My final plan is making auto localization python script which download, extract, modify, and re-compress everything automatically.
A little spoiler -