As far as I know... to change this label you should edit the nautilus Source Code.

NOTE: In this example I'm running Ubuntu 13.04 with nautilus (Files) 3.6.3

1) Make sure you have enable the Source code repository

• Open the Ubuntu Software Center and in the Menu Bar choose Edit -> Software Sources.
  
  Click to enable "Source code repository".

Just in case I use the "Main Server" to Download.

2) Open a Terminal and install the necessary packages.

• sudo apt-get install build-essential

3) Install build dependencies

• sudo apt-get build-dep nautilus

4) Create a folder to download the source code.

• mkdir ~/Downloads/src




• cd ~/Downloads/src

5) Download the source code.

• apt-get source nautilus

6) Edit the file "nautilus-places-sidebar.c"

Search for the line 3261 to edit.

Replace "Computer" for "File System".

To edit:

• gedit nautilus-3.6.3/src/nautilus-places-sidebar.c

BEFORE:

sidebar->hostname = g_strdup (_("Computer"));

AFTER:

sidebar->hostname = g_strdup (_("File System"));

In one command would be:

• sed -i '3261s/Computer/File System/' ~/Downloads/src/nautilus-3.6.3/src/nautilus-places-sidebar.c

7) Go to the "nautilus-3.6.3" folder to build the deb packages.

• cd nautilus-3.6.3/




• dpkg-buildpackage -rfakeroot -uc -b

8) Now you can install the deb packages.

• cd ..




• sudo dpkg -i *deb

9) Finally you can logout and Login to see the changes.

BEFORE:

AFTER:

Hope it helps.