K tomu to vraciam stále a neviem si zapamatať riešenie.

Najprv treba regerovať locales:

root@pi-2:/home/tomas# locale-gen en_US en_US.UTF-8 sk_SK sk_SK.UTF-8
Generating locales (this might take a while)...
 en_GB.UTF-8... done
Generation complete.

Potom treba rekonfikurovať balík locales:

root@pi-2:/home/tomas# dpkg-reconfigure locales
Generating locales (this might take a while)...
 en_GB.UTF-8... done
 sk_SK.UTF-8... done
Generation complete.

A je to hotové.