There is a good chance I'm wrong, but I don't think there is any special kernel option unless you are using a FAT filesystem somewhere and want UTF-8 support on that filesystem. You may be using a FAT filesytem if you are dual booting MS Windows and you may occasionally use one if you have an ipod, thumbdrive, digital camera, etc... You can add support for FAT filesystems in filesystems -> native language support. The stock 2.4 kernel on Slackware compiles this as a module.
You can find instructions on enabling UTF-8 in Slackware here:
In the example, they use en_GB.UTF-8. 'en' is the language code, and 'GB' is the country code. You can find such codes at the following places:
Finally, you may want to add international fonts. If you have access to an XP machine, you can steal them from Windows XP. Another way is to add the dejavu fonts. If you want help with either of these tasks, then the following links will be helpful:
(for XP fonts)