This document is a bit further than my current problem. As far as I understood it's about keys in X.
Thanks for that, bigrigdriver.
I'll be doing this in X after I will do it in console.
Currently I found a way to modify *.map.gz. And, as all in Linux, map.gz appeared to be quite a descriptive file itself.
I can now assign any key on my keyboard to switch language

So I'm thinking now which one to choose
