Hey @juhule,
Currently, the locale will only affect some of the text in the plugin (the Like Box widget, and the "story" text from Facebook - things like "Juhule added a photo"), however, Facebook recently added the ability to load multiple languages via their API using the locale setting - if the Facebook posts exist in more than one language. I'm actually planning to push out an update today (within a few hours here) which will allow you to choose the language using the "locale" setting in the shortcode. Eg:
English:
[custom-facebook-feed locale="en_EN"]
German:
[custom-facebook-feed locale="de_DE"]
Look out for an update later today, and then once you've updated let us know if you still have a problem with this.
Many thanks!
John