Skype should allow users to select display language
Skype should, when installed, give the option to select the display language used to display the User Interface in Skype.
There should be no assumptions made regarding Culture based on this selection (culture is the setting which dictates how time, date, numbers, decimal points, thousand separators and so on should be used).
Either Culture should be picked up from the underlying Operating System or it should have it's own setting.
Enrico Nekrad Weigelt commented
On decent operating systems (eg. Unix world) these things are controlled via environment variables, *NOT* application settings. That way, these things can be easily controlled by the user - even per instance, and the application doesn't even need to have extra code (eg. config dialogs) for that.