I'd like to use the babel-croatian package which is available for download on CTAN. But how do I install this package? Or perhaps there is a command-line way?
Debian's TeXlive packages often group many related CTAN packages. You can browse the list of
texlive-… packages to find the one you want. If you know the specific file name that you're after, for example
croatian.ldf for the Babel language definition file for Croatian, you can look it up on the website or use the
apt-file search croatian.ldf
In wheezy, this file is in
texlive-latex-base (which contains all the language definition files that Debian ships). There is an additional package
texlive-lang-croatian with hyphenation support, documentation and other tools for typesetting Croatian.
The same principle would apply to fonts (assuming they're in Debian) — but Croatian doesn't use a specific font.