You need to have a Qt translation for your new language,
otherwise your language won't be loaded at runtime.

To add a new language:
1) Inside src/sdk/sdk.pro:
   add your language into IB_LANGUAGES and QT_LANGUAGES.
   QT_LANGUAGES contains base name of the Qt translation file.
2) Add Installer Framework translation filename into:
   src/src.pro
3) Add Installer Framework and Qt messages (.qm) filenames into:
   src/sdk/installerbase.qrc

In order to provide an update to translations, according to changes done in sources, run:

lupdate installerfw.pro

in a root directory.