Internationalize log

Since Extension Manager tasks log is displayed to the user we might want to internationalize it some way.

The hard part right now is that most of theses messages are generated in a xwiki-commons module and the new localization module is still in platform.

I can think of several things:

• move part all we can of the localization module to commons (I think we want it anyway at some point) and use it everywhere we log but it's a bit of a pain
• log keys instead of real messages and translate them when displaying the log but it's quite a pain when debugging
• provide the message key with the default message some way and use the key to find a translation when displaying the log
• generate the translation key from the default message but that's very fragile since it's going to break if we change anything in the default message