Works on both platforms:
Quit After Efects.
Create a empty textfile named “ae_force_english.txt” and put it in the documents folder (the one in your home directory)
Start After effects.

Other methods:

Windows:
To force After Effects to run in U.S. English on Windows do the following:

1. At your desktop or your quickstart bar, right click the shortcut of your After Effects installation and select Properties.
2. Under Target, where you see the path to AfterFX.exe, add the following text at the very end, behind quotes if present: -L en_US
3. Relaunch After Effects and run the script again.

Macintosh OS X (10.5 and below):
To force After Effects to run in U.S. English do the following:

1. Quit After Effects.
2. Do a get info (Cmd-i) on the After Effects application.
3. Under the languages section, uncheck all languages except US English.
4. Relaunch After Effects and run the script again.

Macintosh OS X (10.6 and above):
To force After Effects to run in U.S. English do the following:

1. Quit After Effects.
2. Go to Language & Text in the System Preferences panel.
3. Drag English to the top of the list.
4. Relaunch After Effects and run the script again.