Use the correct translation namespace. As we run in gluon config mode,
we do not have our own controller. Thus we need to specify the
translation namespace for gluon to find translations. See
<https://gluon.readthedocs.io/en/stable/dev/web/i18n.html>:

> Note that translations are namespaced: each package will only use its
> own translation strings by default. For this purpose, the package name
> must by specified in a package’s controller. It is possible to access
> a different translation package using the i18n function from models
> and view, which is necessary when strings from the site configuration
> are used, or packages do not have their own controller (which is the
> case for config mode wizard components).
>
>     local site_i18n = i18n 'gluon-site'
>     local msg = site_i18n._translate('gluon-config-mode:welcome')
>