[server] Set language of connection in all cases

If self.lang was already defined (aka not None), the language was not set. In consequence, when a pview was called, the language was always considered to be english, this piece of information is used by gettext, so it resulted in translation always done in english language instead of using user preference as excepted.

• closes #87 (closed)
• clones !82 (merged)

Ça a déjà été validé pour la 3.26. Je fais une demande de fusion pour le lancement des tests sur la 3.29 au cas où ça casserait pour une mystérieuse raison.