Hi Nicolas,
I cannot answer you in French, but as nobody else has done it in a few days I'll go ahead and do it in English.
Language_fr.properties is generated at build time from Language_fr.properties.native using the native2ascii tool that is part of the JDK. The .native one is encoded in UTF-8 and is the one modified to add a new translation or change an existing one. But only Language_fr.properties is read at runtime.
Does this explanation help?
Por favor, identifíquese para poder marcar esto como inapropiado