Some time ago (in WIP phase) English was always preselected along with the other, “main” language (corresponding to installation file name, eg. Polish for “pl-pl”). I think this is a better default option for most users.
As far as I know, to install multiple languages you need to select this option during the installation:
Kelvin told me that when there are extra languages selected, every time you install a new build, the installer downloads all the selected language packs again, and it slows down Rhino installation.
I also added a bug in the bugtracker some time ago so that Language Packs for Rhino 6 can be downloaded from the website, as we now have for Rhino 5.
You can repair the installation and add more languages to the installer; I don’t see any reason to have a separate download mechanism, so I marked that issue as “Won’t Fix”.
I added another language modifying Rhino installation (in Win 10: Apps & features - Rhino 6 - Modify - Modify).
It worked but:
there was a different method of licensing selected: Rhino Account instead of “This computer” (I used originally) - I had to change it and enter license key again
I had to validate the license once again (edit: this might be a different issue as I am asked for validation each time I open Rhino - I will elaborate in different topic or channel)