To clarify, the periods are shown correctly when amule is first opened. But the moment I click the Preferences button in the toolbar, the periods change to commas.
I have checked the amule.conf file and it matches the language setting in the amule 2.13 preferences. In this case, it correctly shows "Language=en_US".
Once the periods change to commas, the only way to change it back is to quit and restart amule. I am using amule 2.13 Universal Binary version for Mac OS Tiger.