What is the purpose of /usr/lib/locale and /usr/share/locale ? It seems that there's some internalization files, but I think I don't need them. I have a hard-disk space shortage and I'd like to erase them. Is there a safe way to erase/shrink these directories ?

The locale file are, as you guessed, related to internationalisation and the character set of your language of choice. Applications, and distro's, sometimes install locale related files, so you'd be better off saving disk space elswhere. Removing locale files may break your applications.

If your just an English speaker and you spot a serb-croat or persian locale you might be safe deleting it but it would be a risk I'd prefer not to take.

The locale files are used to designate your (i.e. the user) locale (for language and time keeping purposes). If your installation has successfully recognized your locale and time zome, you may safely delete all other locales. You may also safely hunt down and delete languange files which you don't use or can't use (not fluent in those languages). I have done so through three different Linux distros, and the only impact I noticed was in web browsing. The languages I deleted did not display properly.
However, since I am not fluent in those languages, I didn't loose anything.