As explained in this article Slackware is by default en English-oriented distro, though you can add non-English support to various applications, such as KDE, LibreOffice, Firefox, etc.., after installation. However there is an unofficial project that has already added 14 other languages [1] to the Slackware installer and Slackware specific manpages (pkgtool(8) and slackpkg(8)). They have all been bundled into the Slint distribution, which is binary compatible with the original Slackware release and Salix.
International support in the OpenSource world is not cohesive. Some proliferated projects have excellent support, Firefox has over 120 language packs, LibreOffice over 110 and KDE more than 50. Popular distributions may also have decent support for non-English languages, in the Debian installer you can choose between 57 languages. But this only applies to the installer, the majority of programs in the Debian repository will only have English documentation (there are only 11 translation of the GNU Manual pages for instance - and some of these are out of date), not to mention the fact that all computer related text books of worth will be in English. In the proprietary world things are simultaneously better and worse (which is often the case). The Apple App Store has 28 languages (and I presume you have to pay for them..?). Windows has over 100 language packs, but their availability is suspect.
During the Windows 7 era for instance, all Windows licenses only supported one language, with one exception: Windows 7 Ultimate Edition. Despite it's name this edition only included two useful features not found in the other versions; BitLocker, a tool for encrypting your harddrive (wait... couldn't the standard version do that?!?). And as mentioned the ability to select a different language. This edition certainly had an Ultimate price tag, if you wanted Windows with English (!) support in Norway, you had to dish out an additional 500 USD for your license. This was a new low, even for Microsoft...
Anyway, enough ranting :^) The reason I am mentioning Slint, is because I wrote the Norwegian translation for Slackware. I worked hard to ensure that the spelling was correct, but being me after all, you should probably expect to find a few spelling errors anyway... In conclusion lets take a few screenshots:
1) The languages are: Dutch, French, German, Greek, Italian, Norwegian, Polish, Portuguese (Brazil), Portuguese (Portugal), Russian, Spanish, Swedish, Turkish and Ukrainian.
Slackware live: The unofficial live version of Slackware current, from Alien Bob, also supports several languages. You can get it here
jw.org: Of course for the ultimate example of translation prowess, see jw.org, which at the time of writing supports nearly 1000 languages!