You could remove/rename the translation file from the install, french would be
... /share/git-gui/lib/msgs/fr.msg
Don't know about OS-X, but under windows (msysgit) that would normally be C:\Program Files\Git\share\git-gui\lib\msgs\, and on Linux (and others) /usr/share/git-gui/lib/msgs/ .
If you don't want to affect anything else except git applications you might add the following line in the beginning of C:\Program Files\Git\cmd\git.cmd file:
@set LANG=en
Please note that this will only work when launching commands from the git shell - GIT GUI launched from the start menu will not be affected
2) Delete or rename relevant *.msg file in C:\Program Files\Git\share\git-gui\lib\msgs
You save on not modifying any setup shell (especially if you use cmd.exe shells) but
you lose on international functionality.
Note that setting @set LANG=en in cmd file helps, but only when you start GitGUI with that cmd file. This is not the case when you start it from the Start menu: it calls wish.exe directly. If you change the link to run cmd script, it shows text command window along with GUI, which is unwanted. That is why for me renaming .msg file is a way of choice.
to start your git-gui for example temporary in english from your current terminal. This solution will affect only your current instance of git-gui and nothing else. Credit goes to Junio C Hamano
Update for use in *.desktop files (persistent solution):
To always start a program with the desired locale setting from your *.desktop file you have to modify it's Exec=... section to start in a modified environment.
In this case you most likely don't have the proper locale generated, yet. (If you see English text nevertheless it's probably your system that's using the C locale as a fallback)