If Extension Manager is launched from Create Suite products' menu, it will use the locale of the product. If Extension Manger is launched in standalone mode( launching it from start menu or by double clicking or command line ), it will use the locale of the operating system. If you do want to run it in another language, you can add "-locale lang=" parameters. For example, on windows, you can run the command below to show French.
"C:\Program Files\Adobe\Adobe Extension Manager CS\Adobe Extension Manager CS.exe" -locale lang="fr_FR".
