This will be in one of the modified ubiquity packages I think (which you can find here: http://packages.linuxmint.com/list.php?release=Petra
). Ubiquity is the name of the installer, developed by Ubuntu. Linux Mint modifies the packages to use Ubiquity for Linux Mint also. So is the translation wrong in the Ubuntu source, or in the Linux Mint modification? In any case, I'm unable to find that string on the computer or in fact to see any locale files for Ubiquity
I'm not sure where this string is gotten from...
I think the translation strings for Czech display of the installer might be here https://translations.launchpad.net/linu ... t/+lang/cs
in the section called live-installer. I've not been able to find the text from your screenshot though