Add a script to translate navbar.mpp

FWIK, there is currently no way to translate headers and footers. This pull-request provides a first solution for that:
* a script scripts/translate.ml takes the filename and a string as arguments, and returns a translation of the string in the language required by the filename extension
* this script is used around some strings in .mpp files to translate them

For every language, the script will have to be modified to add the corresponding translations. This could also be done by making the script read a translation file (TODO for next contributors).