After adding support for additional languages, Regional Settings may display some unreadable characters. Unreadable text may appear in other locations as well.
When you install support for additional languages, files in the Langpack folder enable support for non-native characters in documents and e-mail. The default system font is still used for the normal system interface (and it doesn't support the additional characters).
See Q177561 - HOWTO: Add and Enable Additional Languages in Windows NT.