[-] BO : Fix $gz usage in updateModulesTranslations

In the Language::updateModulesTranslation(), the $GZ is used after a froeach, but the init of this variable is done inside this foreach.
It may happen that the nit is not done, if no loop is done.

For exemple, when Prestashop is used on a local desktop, not conected to the internet, it's impossible to install a new module : the $gz is not initialised in this case