Merge: Opportunity i18n & traduction française

This PR brings partial i18n support to Opportunity with a french translation. Most strings from the
 UI are covered but not the error messages. This PR also fixes a few bugs in the i18n phase.

I still have difficulty with the deployment. Launching my web server use `LANGUAGE=fr sudo bin/xymus_net` works, but not when using `daemonize -E LANGUAGE=fr ...`. @R4PaSs do you have an idea for a solution?

http://xymus.net/opportunity/ temporary serves the French version of the site. I may have to revert tot the English version if I need monit and I can't fix the locale issue.

Pull-Request: #1531
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>

diff --cc lib/standard/text/abstract_text.nit
Simple merge