Translations of the Installer are not applied

Tested on 3.0~rc1, in French and Spanish: no string is translated in the GUI.

Feature Branch: liveusb-creator.git:bugfix/12588-i18n-gtkbuilder

Original created by @intrigeri on 12588 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information