Please don't post the same question in four different forums. It leads to fragmented threads and unnecessary duplication. I have deleted three of the four identical threads.
If, by "language packages", you mean support for more than one locale, the see the Gentoo Linux Localization Guide
. Ignore sections 1 and 2 regarding the time zone and hardware clock, as those assume a system using Baselayout-1 whereas SL uses Baselayout-2 and OpenRC. Therefore for the time zone and hardware clock see the SL Wiki article HOWTO: Clock, Time, UTC, Dual boot with Windows
. For the language support, the rest of that Gentoo guide is applicable.