add i18n for loading message

The message appearing when the application loads has no default translation. This PR fixes this.

By the way I have added by hand the msgid/msgstr in the .pot/.po files. Perhaps it would have been better to use xgettext and so on to regenerate the files but the doc is missing in the developper doc http://docs.camptocamp.net/c2cgeoportal/developer/index.html and I didn't wanted to spent to much time looking for the options to use (for instance how are we supposed to do to retrieve the strings to translate from the scaffold templates?).