DateIntervalMapping i18n

* implement a "since xxx" construct common in Fr
* allow several words per language for "present"
* adding more languages in config file
* custom (textual) separators for DateIntervalMapping (language dependant): allows for "de ... à ..." used in Fr
* create unit tests
maybe it's better to return empty string here or the default "en" version to be consistent (that would be empty here)
this would simplify the `match null` below