Perhaps separate issues should be opened and link as related, but I'll just put lots of stuff here ATM.
At the same time, I am making notes regarding localization on the wiki: https://www.reactos.org/wiki/User:Alvinhochun/Localization
1. intl.cpl: "Geographic Location" combobox is blank on fresh install (at least for en-US and zh-TW), but will act normal after one selects a location and save. (Probably related to usetup not setting it properly.)
2. intl.cpl: Changing "User Locale" ("Standards and formats" in "Regional Options" prop. sheet) incorrectly changes the "Default" and "InstallLanguage" values under HKLM\SYSTEM\CurrentControlSet\Control\Nls\Language, which in turn changed the UI language after a reboot (which is not supposed to happen). It also incorrectly changes the OEM Codepage and ANSI Codepage.
The "User Locale" setting really is only supposed to change the current user locale.
3. intl.cpl: Changing the "System Locale" ("Language for non-Unicode programs") only changes the font substitution.
It is supposed to change HKLM\SYSTEM\CurrentControlSet\Control\Nls\Language\Default (which represents the "System Locale"), the Codepage settings (ACP/OEMCP/MACCP), and also certain font substitution and font linking settings.
4. intl.cpl: Changed locale options doesn't reload correctly before next reboot.
5. intl.cpl: The UI language option, which changes the UI language of the current user, is missing in the "Languages" tab. (In Windows, it is only visible if you have MUI Language Packs installed.)
6. intl.cpl: "Apply all settings to the current user account and to the default user profile" in "Advanced" tab isn't implemented.
7. intl.cpl: Changing "Code page conversion tables" does nothing when saving.
8. intl.cpl: "Additional language support" not implemented (low priority though.)
9. input.cpl: If you chose Chinese (Taiwan) when installing, then in input.cpl you'll see three entries of "Chinese (Taiwan)" with "US" keyboard and an "English" one. This is due to two of them being IMEs and they are not implemented in ReactOS yet. (This is not the bug.) However, when you start removing keyboard layouts, when there are supposed to be two remaining, there will suddenly be only one listed, and the "Keyboard Layout" column will show gibberish.
10. input.cpl: Windows XP uses a tree display for input settings.