After dukeala left a comment telling about a blog by Jaanus on the skype website, I decided this would be better edited to include that link. Using what the blog describes, there is no need for a hex editor of any kind.
As wriiten by dukeala,
"...you do not actually need a Hex editor for that, you can simply create, edit and load your own language file - you can read about it at http://share.skype.com/sites/en/2006/11/customizing_skype_with_a_hex_e.html "
As you can see, instead of going through all the trouble using a hex editor, there is an easier way to do it.
While it is almost certainly against the Skype ToS to do so, I thought I would post this as a proof of concept. Do not do this, as it is against the ToS. This is offered for educational use only.
Also, this is my first Instructable, so feel free to rip me a new one in comments ;-)
Step 1: Obtain Needed Software.
A Hex Editor: I used http://www.chmaas.handshake.de/delphi/freeware/xvi32/xvi32.htm